Over recent years, opinions about global warming have become increasingly polarised.

It came to a head late last year when hundreds of e-mails from the University of East Anglia's Climatic Research Unit were published.

The so-called "Climategate" debate was born.

Despite governments, scientists and campaigners telling us the world's climate is changing, opinion polls suggest growing uncertainty about global warming.

It hasn't helped that recent weather forecasts of a barbecue summer and a mild winter have been spectacularly wrong.

Panorama's Tom Heap has gone back to basics to ask what we really know about our climate and how it affect us.

His examination of the topic comes just ahead of the third and final report into "Climategate".

Thus far, there have been two inquiries: the Parliamentary Science and Technology Select Committee and an independent review by Lord Oxburgh.

Both found that there was no grand global conspiracy and no deliberate scientific impropriety or dishonesty.

The final report by Sir Muir Russell has looked at whether the scientists involved could have been more open with their critics.

The e-mails, which talk of hiding and deletion, gave some the impression that information had been deliberately withheld. They also appeared to lift the lid on an apparently vicious and personal conflict.

We wanted to pin down what, if anything, is broadly agreed and certain about global warming.

Top Gear may have its "Cool Wall", but we have built a "Wall of Certainty". We tested it out on some leading scientists and asked them a few key questions about climate change.

We gave them just four options - certain, likely, unlikely or no way. The answers were fascinating.

First up was Professor Bob Watson. He chaired the Intergovernmental Panel on Climate Change for five years, worked for former US Vice-President Al Gore, and is now chief scientific adviser to the Department for Environment, Food and Rural Affairs.

Second was the leading sceptic Professor John Christy, an atmospheric scientist at the University of Alabama in Huntsville.

Contrary to some of the newspaper headlines and blogs that suggest all global warming science is a con, they agreed that mankind is causing the planet to warm up.

We also hear from the scientist behind the graph which has become an icon in the climate-change debate. Professor Michael Mann regrets the way his so-called "hockey-stick graph" was put in the spotlight by politicians.

And we find out that the leading sceptic Bjorn Lomborg, author of the best-selling book The Skeptical Environmentalist, accepts much of the basic science and agrees with the critical IPCC finding that most of the recent global warming is man-made.

There is genuine uncertainty and disagreement about the exact scale and speed of human-induced global warming and crucially what we should do about it. But I was surprised to find how much agreement there is on the fundamental science.

Mike Rudin is producer of Panorama's What's Up With The Weather? You can watch it on BBC One on Monday 28 June at 2030BST and online.

What is immediately apparent is the lack of intelligence about Bin Laden. We hear from the man who was tasked by President Obama to review US policy in Afghanistan and Pakistan.

The former CIA officer Bruce Riedel has seen the intelligence on Bin Laden and his blunt conclusion is that "there is no trail any more".

"It's not cold," he says, "it's frozen over".

Riedel, who has advised three US presidents, admits that despite the biggest manhunt in history "we haven't had eyes on target now in over eight years... and we don't have a clue where he is."

Nowadays, if there's a situation where there's no certainty, in jump the conspiracy theories.

Over the last eight years, Osama Bin Laden has become shrouded in myth and rumour.

There have been reports from some leading news organisations suggesting Bin Laden has been seriously ill with kidney disease and even some claiming that he is dead.

The leading French newspaper, Le Figaro, and Radio France International reported that Osama Bin Laden was given emergency kidney dialysis in the American Hospital in Dubai, a few months before 9/11.

CBS ran a story on their main evening news suggesting that Bin Laden was given kidney dialysis at the Pakistan Military Hospital in Rawalpindi, on the day before 9/11.

Often these reports emanate from smaller newspapers, such as one in the Pakistan Observer, which claimed that Osama Bin Laden had died of a lung complication during the battle for Tora Bora at the end of 2001 and was buried there in the mountains of eastern Afghanistan. That report was then picked up around the world.

Such stories might once have slowly faded from view but now the internet stores them and endlessly recycles them. Years on, the stories are presented as fact, without any checks.

The hospitals in Dubai and Pakistan both deny the stories without reservation. Both have investigated the reports and checked their records.

Robert Baer spent 21 years working for the CIA as a case officer in the Middle East, with postings in Sudan, Lebanon and Iraq. He is surprised by how little is known about the world's most infamous terrorist but he's scathing about the reports of Bin Laden's kidney illness.

"He's probably in the same bed next to Elvis Presley. You can't hide something like that in Dubai," says Baer.

"It's the crap you read on the internet that people believe in," adds the experienced former CIA officer.

The editor of the Arabic newspaper Al Quds, Abdel Bari Atwan, interviewed Bin Laden in Tora Bora back in the 90s. He is critical of those who stay in their office and don't get out to research stories about Osama Bin Laden and al-Qaeda.

But there's an immediate rejoinder from the sceptics. David Ray Griffin, a retired philosophy professor in California, has written a book suggesting there is good evidence that Bin Laden is dead. He questions the motives of officials and former officials who contradict his theory:

"We do know historically that people have lied under pressure or promise of money or whatever. I'm not making any accusation. I'm saying I don't know these people. I don't know their motivations, but I'm saying that the very fact that people say these things don't necessarily mean they're true."

Griffin's previous book suggested the American government could be responsible for 9/11.

Now he speculates that US military intelligence could be faking Bin Laden's recent video statements to keep an evil bogeyman alive and to help justify the so-called war on terror in Afghanistan, Iraq and back at home.

He thinks the "military-industrial complex" has ample motive for this grand conspiracy in the huge sums that have been spent. Afghanistan alone has cost the US $240bn.

The CIA case officer, Robert Baer, dismisses those suggestions of conspiracy theories involving the US intelligence, but he does question whether Bin Laden is still alive: "The problem is nobody's convinced me he's alive."

Baer questions the veracity of some of the recent Bin Laden videos. Instead though, he thinks it is in the interests of al-Qaeda to fake the tapes to pretend their icon is still alive.

But perhaps the last word should go to the last journalist to interview Osama Bin Laden, back in November 2001.

Hamid Mir, now the executive editor of Geo TV in Islamabad, says Bin Laden is still alive and the secret of his survival is that he is "much cleverer and wiser than the American intelligence". And he warns: "we should not underestimate him. All these conspiracy theories are actually helping him."

Mike Rudin is series producer of The Conspiracy Files. The Conspiracy Files: Osama Bin Laden - Dead or Alive? is on Sunday 10 January at 9.30pm on BBC Two.

But I also think it is important to investigate the conspiracy theories that continue to develop around 7 July attacks, because they play on the fears of the Muslim community and spread a highly divisive and damaging message. The programme carefully and analytically works through the allegations and the evidence to separate fact from fiction.

There have been three official reports into the bombings. However, a host of internet films continue to scrutinise every word and every picture for signs of a hidden truth.

The programme, to be shown on BBC Two at 9pm on Tuesday 30 June shows that on one occasion one sceptic was right and spotted a significant error in the Home Office narrative. The government had to apologise for suggesting in a report, nearly a year after the attacks, that the four bombers had boarded a train which had actually been cancelled.

However, crucially the government insists the bombers were still able to get to London on time, because they caught an earlier train, which was delayed leaving Luton.

Internet videos question the official account, suggesting the British government has deceived people into thinking four suicide bombers carried out the attacks. Some go even further and allege the British government was involved.

The latest Conspiracy Files programme films one notorious conspiracy video being played at the Birmingham Central Mosque and sees first hand how conspiracy theories have found favour among some Muslims.

One opinion poll by Gfk NOP for Channel 4, two years after 7 July attacks, found that around a quarter of British Muslims questioned thought the government or MI5 were involved in the bombings.

Rachel North, who survived the bomb on the Piccadilly line, tells the programme that the conspiracy theories need to be countered for that very reason:

"If people in mosques think that the Government is so antagonistic towards them that they're actually willing to frame them for a monstrous crime they didn't commit what does that do to levels of trust? That is a problem for the government and for everybody in this country."

Brian Paddick, who was Metropolitan Police Deputy Assistant Commissioner at the time of 7 July 2005, argues it is important to counteract the conspiracy theories:

"Programmes like this may be very controversial but hopefully there will be people in the police service and in the security service and in government who will realize how important conspiracy theories are. And how important it is to try and prevent further atrocities that every attempt is made to try and counteract them."

Mike Rudin is series producer of The Conspiracy Files. The Conspiracy Files: 7/7
is on Tuesday 30 June at 9pm on BBC Two.

What I didn't know was how relaxed the British public has been about the collection of data and how pervasive surveillance has become.

The office of the Information Commissioner runs an opinion poll every year asking people to rank various social concerns such as crime, health, unemployment and the environment.

Last year, for the first time ever, the report [361Kb PDF] shows the protection of personal information ranked equal alongside concerns about crime. Three years previously [193Kb PDF], it ranked third.

The Information Commissioner, Richard Thomas, told us that he thinks that people have now "woken up to the implications" of increasing surveillance.

So maybe things are changing. But we already have a huge network of surveillance. The big change has been the growth of enormous databases which not only record masses of information but also allow it to be searched, linked and accessed by other systems. Increasingly, modern surveillance is general and not targeted.

Take the network of number plate recognition cameras spreading across the country: a system that reads everyone's journey and holds it for two years. Of course, this is aimed at criminals and terrorists, and it has an important function - but to get them, it holds records on all of us.

And it's not just the government that's watching us; business is too. Look at all those loyalty cards that we willingly use, which record everything we buy. And look at some of Google's latest features: Latitude - where, if you opt in, others can track your location based on your mobile phone - and Street View, which allows you to look in detail around cities, but which also filmed lots of people in places where they would rather not have been seen.

A surprising number of CCTV cameras are now provided online, openly and publicly. For the first time, we've started to put all those cameras on a clickable map of the UK. We want to build the map to show just how much is already available. So, if you spot a CCTV camera that has a public feed, please go to bbc.co.uk/whoswatchingyou and add it to the map.

This is, of course, just the tip of the iceberg. The official figures suggest that there are around 30,000 CCTV cameras operated by public authorities. Beyond that, there are hundreds of thousands - and almost certainly millions - of private CCTV cameras.

It's worth looking around to see all the unusual places where cameras have now been installed. We're also asking for your pictures of surveillance in the UK - where's the most unlikely camera?

Privacy seems to matter little until it's your privacy that's being invaded. Most of the time, surveillance is hidden and we're unaware and unconcerned about just how much there is.

Who's Watching You? delves into that secretive world to see just how much surveillance there is and what it means for all of us - and your contributions are appreciated.

Mike Rudin is series producer of Who's Watching You? and The Conspiracy Files. Who's Watching You? starts on Monday 25th May at 9pm on BBC Two and BBC iPlayer.

BBC special correspondent Richard Bilton has been taking a journey through our surveillance society for a new BBC2 series called "Who's Watching You?", which will be broadcast this spring. He's been meeting both the watchers and the watched.

Cheaper and more advanced technology has prompted a massive expansion in surveillance - not just through CCTV, listening devices, tracking, but also through all the personal data that's collected on every one of us.

As the Information Commissioner Richard Thomas says, we leave an "electronic footprint" behind us almost wherever we go - with every click of the mouse, every phone call, every time we use a credit card. And that information just grows and grows, allowing a more and more detailed and intrusive picture to be constructed of how we each live our lives.

The paradox is that there is a great deal of support for things like CCTV. We all benefit from better crime detection and from easier and cheaper services. But we know surprisingly little about the depth and breadth of modern surveillance, or about the potential problems when things go wrong.

Wherever we went, we were told: "If you've got nothing to hide, then you've got nothing to fear." But when we looked further, we found people who had suffered from the loss of personal data, and ordinary people who are watched every minute of the day at work and even at home.

Pressure groups like Liberty, Privacy International and NO2ID have long warned of the dangers to personal privacy. But even the word privacy is hard to define and hard to relate to.

What we have found throughout making the series, almost whatever the type of surveillance, is that regulation is all too often an after-thought and sometimes non-existent.

Take CCTV for example. The Lords committee's report makes clear that "there are few restrictions on the use of public area CCTV cameras in the UK".

The government has already announced consultations on creating a new "super-database" to record the fact (but not the content) of every email, phone call and internet use, and also on the use of covert surveillance by local authorities; and it is due to reveal how it will come into line with a recent European Court of Human Rights ruling that our present DNA database has been in breach of the right to privacy.

"Who's Watching You?" will examine how surveillance is now becoming all pervasive but how little we understand it.

Mike Rudin is series producer of Who's Watching You? and The Conspiracy Files.

Almost inevitably I've been enmeshed in the ever growing net of the conspiracy theory. They've added my name to a long list of imagined conspirators - the secret services, police, people who worked in the building, first responders, the fire service, city officials...and also those who they think have deliberately set out to cover up this huge conspiracy - the official investigators, the world's media...

Last month we were in New York to film the seventh anniversary of 9/11 at Ground Zero for a new programme about the allegation of a conspiracy to deliberately destroy the three skyscrapers at the World Trade Centre. "The Conspiracy Files: 9/11 - The Truth Behind The Third Tower" is to be broadcast at 9pm on BBC Two on Sunday 26th October 2008.

We also filmed self-styled truthers who think 9/11 was an inside job, either carried out or allowed by the US government; and they needed to destroy a third tower at the World Trade Center, Tower 7, which they think contained the plans for the plot.

It is a fact that Tower 7 had some interesting tenants - the CIA, the Secret Service, the Department of Defense and the Office of Emergency Management - the very office which was intended to co-ordinate a response to a disaster or terrorist attack.

When we were filming we were surprised that some of the truthers seemed particularly keen to interview us on camera about the last programme we made about this third tower at 9/11. They think we have deliberately set out to conceal the truth. As one said to me "You already know the truth."

The group who made the video are called "We are change". They claim we misrepresented the chronology involving one important witness who we interviewed in our last programme about 9/11.

The first responder Barry Jennings was trapped inside the building for several hours along with another New York City official. They were crucial witnesses to what was going on inside Tower 7 after everyone had been evacuated shortly after the Twin Towers were hit by the two planes.

As I tried to explain to them at the time, we recorded a long interview with Barry Jennings. We also carefully considered other information and came to our own view based on all of that.

As the two men tried to get out of the skyscraper they were suddenly thrown into darkness. Barry Jennings said he heard explosions. We think it is likely that this was when Tower 1 collapsed, showering debris onto Tower 7.

We have also recently recorded an interview with the other man there, Michael Hess.

Michael Hess was Mayor Rudolf Giuliani's chief lawyer, in charge of 800 New York City lawyers. In his first interview since 9/11 he confirms our timeline. Hess says all the lights went out and he felt the building shake like an earthquake and he adds that he did not hear explosions.

In his mind he thought there might have been an explosion. In the only interview he did on 9/11 he told a reporter he had "walked down to the eighth floor where there was an explosion."

But as our interview with him shows, he is now certain that he did not hear an explosion. He just assumed on the day it could have been an explosion because he had witnessed the lights going out, the staircase filling with smoke and the building shaking vigorously.

We now know, courtesy of the final official report on 9/11 [pdf link] by the National Institute of Standards and Technology, that the official investigators think that two areas of Tower 7 were badly hit when the 1,350 foot Tower 1 collapsed. Seven columns were severed on the southwest corner and they suggest debris also hit the top centre face of Tower 7.

The lead investigator of NIST told me that "it's likely that all of those huge failures and damage really caused noises that were incredibly loud."

If our timeline is wrong then why didn't Barry Jennings and Michael Hess see and hear the moment of impact when Tower 1 fell. It must have been very loud.

The group also criticizes us for not including one sentence from an interview with the owner Larry Silverstein. Apologies now because this gets very complicated. However, some people think hidden in this is a vital clue that can unravel the biggest conspiracy in modern times.

The theory is that the owner Larry Silverstein is meant to have implicated himself in a conspiracy to destroy the buildings he owned and leased at the World Trade Center. And what exactly did he say that supposedly gave it all away?

He said "pull it" which some people interpret as an order to demolish the building.

The interview was conducted the year after 9/11 and Larry Silverstein said:

"I remember getting a call from the fire department commander, telling me that they were not sure they were gonna be able to contain the fire, and I said, "We've had such terrible loss of life, maybe the smartest thing to do is pull it."

"We are change" activists think there is something sinister in the fact that we did not include an extra sentence when Larry Silverstein said:

"And they made that decision to pull and we watched the building collapse."

I don't have a problem talking about it. And just for the avoidance of any doubt we included it in the Worldwide version and we will include it in the new programme for BBC Two.

However, I do not understand how that implicates the owner in any wrongdoing. Interestingly one prominent website, 9/11 Research which is highly critical of the official explanation, is not convinced either and thinks it might even be "bait" to discredit the truth movement.

The crucial words seem to be "pull it" and Larry Silverstein's spokesman provides an explanation:

"Mr Silverstein expressed his view that the most important thing was to protect the safety of those fire fighters, including, if necessary, to have them withdraw from the building."

I talked to the man who assumed command of the New York Fire Department that day. Chief Daniel Nigro told me that it was his decision to decide what to do about Tower 7.

In other words there is no way Larry Silverstein could have ordered the fire department to leave the building and wait for it to be demolished. As Chief Nigro told me the fire service was not part of conspiracy, they were doing their job:

"When we are in charge of a building, we are in charge and that decision will be the fire chiefs and his alone...That's why I know there is no conspiracy, because for me to be part of that would be obscene and it disgusts me to even think of it."

For good measure the truthers at Ground Zero added one final criticism, saying that the BBC is funded by General Electric. I'm not sure what that's based on, but I can say with absolute, yes absolute certainty, it isn't...nor have I ever been part of a conspiracy to cover up what really went on 9/11.

Speaking at the NIST headquarters just outside Washington DC, Dr Shyam Sunder said it was definitely not a controlled demolition, as many self-styled "9/11 Truthers" maintain.

After three years extensive and exhaustive investigation NIST has published their final report on the World Trade Center and concluded that fire caused Tower 7 to collapse (pdf link). And they deduced from a series of highly complex computer models that the collapse started with the failure of just one column, column 79 on the 13th floor.

Dr Sunder added that the collapse of WTC7 represents the first known instance of the total collapse of a tall building primarily due to fires.

Why was it the first in the world? NIST scientists say it was because fires were allowed to burn in Tower 7 for nearly seven hours and the mains water had been severed by the collapse of WTC1, meaning that fire fighters were unable to fight the fires. The failure of the mains water supply also cut the sprinkler system to the lower half of the building, where the fires burnt for longer and more fiercely.

NIST specifically looked at whether controlled demolition using conventional explosives could have caused the collapse. But they found that the minimum charge necessary to destroy just one key column would have produced a huge amount of noise - between 130-140 Db even one kilometre away from WTC7 - the equivalent of standing next to a jet plane engine.

Yet NIST found no witnesses who had heard anything like that, nor any such noise on any of the videos of the collapse and none of the shattered windows that would have been expected on the backs of buildings.

Dr Sunder told me that they judged other possible hypotheses, such as the theory that there was a controlled demolition using an incendiary called thermite or thermate, were "not credible enough to justify investigation".

Yesterday was the closing day for comments on that final official report.

We have been filming a new set of interviews for an update of "The Conspiracy Files: 9/11 - The Third Tower" to be broadcast on BBC Two soon. With the publication of the final official report on 9/11, officials think the long process of investigation is at last nearly over.

The more high profile and outrageous the act the more difficult to keep it secret. Richard Clarke added that in all the 30 years he'd had top level intelligence clearance nearly all the key secrets he'd seen had become public.

The former counter-terrorism adviser is adamant that the theories that a third tower, called World Trade Center Building 7, was brought down by a controlled demolition is not possible:

"Hundreds of people thousands of people would have had to have been involved and know about it and it most surely would have leaked out."

One of the comments on the blog raised the Iran Contra conspiracy as evidence that in fact you don't need hundreds or thousands of people to carry out a very complex conspiracy. Iran Contra is a good example because it shows clearly that conspiracies do happen. They can be complex.

But the Iran Contra conspiracy was exposed. And what's more, as it went on it necessarily began to drag in more and more people.

Richard Clarke also made the point that any conspiracy on 9/11 would have been unbelievably risky for Bush or Cheney:

"The risk to them would be personally, that they would go to jail for the rest of their lives. That they would have their name blackened throughout history, and that their political party would be destroyed. Everything that they had ever worked for their party for their family, for themselves, would be utterly smashed."

Other comments have suggested that it is wrong to mention that critics have suggested that the BBC and other sections of the media were allegedly involved in a conspiracy. If you read the many comments on this blog it is clear that people are suggesting just that.

Yet once again those suggestions draw in hundreds and thousands more people. Every time more people are drawn in it makes the chances of keeping anything secret even more difficult.

The long awaited final report on Tower 7 at the World Trade Center is due later this month and the American government hopes it will provide the answers to the questions that have been raised about that building.

This is likely to be the government's last word on 9/11. Those who continue to question the official explanation have still to set out what exactly the grand conspiracy was, how it was carried out, who carried it out and why they would do so.

The Conspiracy Files: 9/11 - The Third Tower repeated on Tuesday 8 July at 2320 BST BBC Two

For the latest Conspiracy Files programme, on 9/11 - The Third Tower (Sunday 6 July at 2100 BST on BBC Two), I've been looking in detail at allegations that there was a conspiracy to deliberately demolish a third tower at the World Trade Center.

This third skyscraper was never hit by an aeroplane. There is little photographic evidence of extensive damage. Yet seven hours after the Twin Towers collapsed, this 47-storey building collapsed in a few seconds.

Afterwards the thousands of tonnes of steel from the building were taken away to be melted down in the Far East. The official explanation is that this third huge tower at the World Trade Center collapsed because of ordinary fires - but that makes this the first and only skyscraper in the world to have collapsed because of fire. Nearly seven years on the final official report on the building has still not been published. The report is now promised this month.

World Trade Center Building 7 has become the subject of heated speculation and a host of conspiracy theories suggesting it was brought down by a controlled demolition. And some people suggest it was not just the government and foreign intelligence, but the police, the fire service, first responders and even the media that were involved.

It is certainly true that on 9/11 the BBC broadcast that WTC7 had collapsed when it was still standing. Then the satellite transmission seemed to cut out mysteriously when the correspondent was still talking. Then Richard Porter admitted in his blog last year that the BBC had lost those key tapes of BBC World News output from the day.

So is that proof that we at the BBC are part of a huge sinister conspiracy or is there a simpler explanation?

The mystery of the missing tapes didn't last that long. One very experienced film librarian kindly agreed to have another look for us one night. There are more than a quarter of a million tapes just in the Fast Store basement at Television Centre. The next morning I got a call to say the tapes had been found. They'd just been put back on the wrong shelf - 2002 rather than 2001. Not so sinister after all.

What about the incorrect reporting of the collapse of Tower 7? Having talked to key eyewitnesses who were actually at Ground Zero that day it is clear that, as early as midday, the fire service feared that Tower 7 might collapse. This information then reached reporters on the scene and was eventually picked up by the international media.

The internet movie Loose Change has been viewed by more than 100 million people according to its makers and it asks this question in the latest film release: "Where did CNN and the BBC get their information especially considering the building was still standing directly behind their reporters?"

It turns out that the respected news agency Reuters picked up an incorrect report and passed it on. They have issued this statement:

"On 11 September 2001 Reuters incorrectly reported that one of the buildings at the New York World Trade Center, 7WTC, had collapsed before it actually did. The report was picked up from a local news story and was withdrawn as soon as it emerged that the building had not fallen."

I put this to the writer and director of Loose Change, Dylan Avery. I asked whether he believed the BBC was part of the conspiracy. Given the question his film had posed about the BBC I was surprised by Dylan's response: "Of course not, that's ludicrous. Why would the BBC be part of it?"

He added candidly: "I didn't really want to put that line in the movie."

And the reason the interview with the BBC correspondent, Jane Standley, ended so abruptly? The satellite feed had an electronic timer, which cut out at 1715 exactly.

We've done our best to tackle many of the other questions raised about Tower 7. I interviewed the lead official investigators, scientists and eyewitnesses who support the official explanation; but also architects, engineers and others who now question that account.

The final report on 9/11 should be with us soon. The official investigators are confident they will be able to solve the final mystery of 9/11. But I doubt they will ever convince their harshest critics, who believe there was a home-grown conspiracy at work that day.

What we will do is investigate an issue. For the new series we have looked for key proponents of alternative theories.

So for the new programme about World Trade Center Building 7 (The Conspiracy Files: 9/11 - The Third Tower for next Sunday) we have interviewed at length the architect Richard Gage, the former professor of physics Steven Jones and the writer of Loose Change Dylan Avery.

We have then taken their questions and arguments and tested them.

We've looked for new photographic and physical evidence, for key eyewitnesses and spoken to experts and investigators who have been involved in trying to understand what exactly happened to bring down Tower 7.

It does matter that a lot of people think the US Government is "hiding something" about 9/11. According to one American poll more than a third of those questioned thought government officials either assisted in the 9/11 attacks or allowed them to happen.

And it does matter that according to the official explanation Tower 7 was the first skyscraper to collapse because of fire. Smaller buildings have collapsed due to fire but never a 47-storey skyscraper.

The final official report on 9/11 by the National Institute of Standards and Technology is eagerly awaited not just by critics but also by those who want to know how safe skyscrapers are.

I'm happy to debate the issues. In next week's programme we will look at the what some people have said was the neat symmetrical collapse of Tower 7, we will look at the dust found around Ground Zero, we will look at the BBC's alleged involvement in a conspiracy, and many other issues.

But I've seen there's already a campaign for letters of complaint well before the programme has been aired.

Alex Jones' Prison Planet website ended an article headlined BBC Hit Piece by urging readers to comment on this blog. And comments in 911blogger.com urged people to prepare a "counter strike" and to start letter writing and e-mailing. A lot of the later comments on my last blog came soon after those.

It would be good if people watched the programme first. So far we've put out a three minute trailer:

In response to dotconnect: yes I'm interested in investigating a host of issues such as the death of Anna Politkovskaya, the financing of al-Qaeda, British agents in Northern Ireland - and it does not as you suggest hinge on whether "our side" was allegedly "behind it". But the BBC has already covered these stories and is currently investigating many of them.

In response to cyncastical: the original allegation made in the papers was that we had paid Nicholas Kollerstrom to appear in the programme about 7/7. We did not. We reimbursed him for £30 worth of his expenses. The newspapers corrected their original copy.

The Conspiracy Files: 9/11 - The Third Tower to be broadcast on BBC Two at 2100 BST on Sunday 6 July, repeated on BBC 2 at 1120 BST on Tuesday 8 July, and on Signzone at 0130 BST on Wednesday 9 July.

But were you to believe what some publications have recently written about our documentary (eg the Sun, the Daily Mail, the Spectator and others) you would be forgiven for thinking that we shouldn't be making the programme at all.

But conspiracy theories about the London bombings are an important public issue.

The stakes are high because conspiracy theories are spreading suspicion about the official account of what happened, ultimately questioning whether the authorities can be trusted. Establishing whether what is argued is true or false, and scrutinising the way proponents conduct themselves, is clearly in the public interest and is a serious and legitimate task for the BBC.

Last year one opinion poll found that around one in four British Muslims do not believe that the four men identified as the 7/7 bombers by the authorities actually carried out the attacks. It is perhaps not surprising that the Metropolitan Police themselves have acknowledged the importance of tackling conspiracy theories about 7/7.

As programme makers we need to be sensitive to the feelings of the families of those who were killed in the bombings and to the survivors. But this should not stop us scrutinising conspiracy theories and the effect they are having on public confidence in the police and the government. Without such scrutiny, these theories are often treated as facts by those who find them seductive.

Some newspapers have alleged that we paid a conspiracy theorist, Nick Kollerstrom, to take part in the programme. This is not true. The BBC has covered the cost of some incidental expenses amounting to no more than £30. This includes the cost of a return train ticket from London to Luton because we asked him to film with us at the location where he had discovered a fact about the bombings - namely that the train that it had been said the bombers took to London did not run on 7 July 2005.

The BBC has also covered the cost of lunch and cups of tea on some of the days we have filmed with him. We did not cover the cost of his trip to Leeds during which he visited the family homes of some of the bombers. Along with his views of 7/7, Nick Kollerstrom's views about the Holocaust will be scrutinised and challenged in the programme.

When the documentary is broadcast in the Autumn you will have the chance to decide for yourself what the facts are about the 7/7 conspiracy theories and the theorists who promote them.

Ten years for officials to try to get to terms with a new phenomenon a truly modern conspiracy theory – developed on the internet, relayed on the mass media and eagerly consumed around the world.

We’ve had the initial two-year French judicial inquiry, then Lord Stevens’ Metropolitan Policy inquiry, Operation Paget, at a cost of £3.7m and now an inquest over six months and costing on a conservative estimate another £3.6m. The total cost to British taxpayers of investigating Princess Diana's death is expected to exceed £10m.

Lord Stevens said he hoped the clear verdict that Princess Diana and Dodi Al Fayed were unlawfully killed due to the "gross negligence" of driver Henri Paul and the paparazzi could bring “closure to what has been a traumatic event”.

Will the inquest verdict finally end the speculation? I doubt it.

Last night Mohamed Al Fayed refused to accept the verdict. He said both the French and the Metropolitan police inquiries were wrong and he insisted that Diana was murdered: "I'm not the only person who says they were murdered. Diana predicted she would be murdered and how it would happen.”

As tonight’s Conspiracy Files Special, produced by Diana Martin, shows new evidence has helped resolve some of the key questions.

For example, it's confirmed there definitely was a second car, a white Fiat Uno, which collided with the Princess’s Mercedes; all the evidence suggests Diana was not pregnant; and it's now acknowledged that her driver Henri Paul had definitely been drinking that night - he ordered and drank two Ricards in the Ritz bar, the equivalent of three measures of whisky.

The coroner said the inquest had served "an important purpose" by examining the conspiracy theories "in minute detail" through the evidence of more than 250 witnesses. Lord Justice Scott Baker concluded that there “is not a shred of evidence” to support the theory that Princess Diana was assassinated by MI6 or any other government agency. But with many important French witnesses refusing to appear before the inquest, some questions will remain unanswered.

Forensic scientists reviewing the toxicological evidence have not been able to explain high levels of carbon monoxide in Henri Paul’s blood samples - which some people claim is evidence that the samples were switched. And a key witness, the driver of the white Fiat Uno has still not been identified.

The investigative journalist Gerald Posner puts that down to the faults of the initial French inquiry. But he tells the programme conspiracy theories have a life of their own:

"When you present solid and credible evidence to somebody who has embraced a conspiracy theory it is extremely difficult to have them give up on their belief. They will claim the evidence you presented has been planted, tampered with, faked by the conspirators themselves. It’s almost impossible to get someone to change their minds."

So was it all necessary? Well yes if so many doubts persist about such a public figure.

We now have a verdict that Princess Diana and Dodi Al Fayed were unlawfully killed due to the actions of driver Henri Paul and the pursuing paparazzi. But should it all have taken ten years to get to this stage?

Of course officials in the UK are not alone in having to deal with counter theories.

In the United States nearly seven years on from 9/11 and yet despite tens of millions of dollars being spent on official inquiries the debate about what really happened on 11 September 2001 continues.

The final, or so it’s planned to be, official report has still to be published and it is due out this summer.

The subject is a third tower that collapsed that day. The 610ft (186m), 47-storey skyscraper collapsed in a few seconds but it was never hit by a plane. According to the official investigators it is the first and only skyscraper in the world to have collapsed solely due to fire.

Later this spring on BBC Two, The Conspiracy Files will report on World Trade Centre Building 7 – a building that has become a rallying cry for those who question the official account of what happened on 9/11.

How Diana Died: A Conspiracy Files Special will be broadcast on Tuesday 8 April 2008 at 1900 BST on BBC Two.

But then came an even more outlandish conspiracy theory suggesting there were two versions of the 9/11 programme which was broadcast last Sunday. Conspiracy piles on conspiracy.

Ian Crane, Chairman of the 9/11 Truth Campaign for the UK and Ireland, claimed last Friday that a source had told him that we were in a “in a quandary over which version of 9/11: The Conspiracy Files will be put out to air”.

He alleged: ”One version is a well-balanced piece of investigative journalism, whereas the alternative version is a hit-piece, intent on portraying 9/11 Truth Campaigners as nothing more than a lunatic fringe group.”

And the story was picked up on the Alex Jones’ website Prison Planet with the headline “BBC Pressured to Air 9/11 Hit Piece?”

Only trouble is there weren’t two versions, no-one bothered to check with us and, what's more, we worked very hard to make sure the programme was fair and balanced.

Behind it all there seems to be a concern that we wouldn’t run a story supporting a conspiracy theory if we found convincing evidence. That couldn’t be further from the truth.

First, there was no editorial interference in the programme whatsoever. Second, if we had found convincing evidence of a conspiracy before 9/11 no one could have held us back from broadcasting such an important story.

We didn’t find anything conclusive proving the conspiracy theories. Instead we found a lot of evidence which supported the official version and contradicted the various conspiracy theories.

Where there was some evidence of a conspiracy after the event to cover-up intelligence failures, we included that in the programme, together with an interview with Senator Bob Graham, who co-chaired a Congressional Inquiry into 9/11.

I know the 9/11 Truth Campaign in the UK and Prison Planet in the USA, among others, are encouraging their supporters to write in. And it’s great to see so many comments on the blog. They make fascinating reading and contain a lot of interesting information.

However, our opinion poll carried out by GfK NOP did not find much support for the underlying conspiracy theory. In a telephone poll of a 1000 adults we asked:

“Attacks were made on the World Trade Centre and the Pentagon on September 11th 2001, commonly known as 9/11. It is generally accepted that these attacks were carried out by ’Al Qaeda’, however some people have suggested there was a wider conspiracy that included the American Government. Do you, yourself, believe that there was a wider conspiracy, or not?”

16% people believed the American Government was involved in a wider conspiracy as against 64% of those questioned who did not believe that. The rest said they did not know.

In fact our opinion poll found much more widespread doubts of the official accounts of the deaths of Princess Diana and the British Government scientist Dr David Kelly. Almost one in three (31%) people questioned believed the car crash that killed Princess Diana was not an accident, 43% agreed it was an accident, and the rest did not know. Almost one in four (23%) people questioned believed the government scientist Dr Kelly did not commit suicide as against 39% who believed he did commit suicide, with the rest unsure.

And this Sunday, The Conspiracy Files series will examine the many questions that surround the death of Dr David Kelly and reveals new material that challenges the official account of his death.

There should be no escaping the fact that nearly three thousand innocent people lost their lives that day.

But I don’t think that respect for the victims and their relatives is a reason to avoid reporting why so many innocent people lost their lives.

It does mean that we should report sensitively.

We hope we have done so in our film about 9/11. And we hear from the sister of a man who lost his life at the World Trade Centre. It is important that the voice of people such as Cheryl Shames is heard.

You cannot ignore the fact that there are a lot of questions about what exactly happened on 9/11.

Chris Townsend makes the valid point that the sheer number of conspiracies should not be mistaken for the depth and breadth of evidence supporting them.

But again I don’t think that is a reason to avoid investigating the questions that have been raised. In fact I think the sheer number of questions is one reason why they should be properly investigated.

I think that any subsequent investigation should be based firmly on the evidence. And we have tried wherever possible to speak to eyewitnesses and to find hard documentary evidence.

Have a look at our Question and Answer section on our website, where we have set out the arguments for and against, and the evidence we have found.

You’ll have to watch on Sunday and make your own mind up how well we answered the many questions that have been raised.

Why was the hole in the Pentagon wall so small and why was there so little damage to the outer wall?

Why does it look like there is no plane at the crash site in Pennsylvania where flight United 93 came down?

Why did a building called World Trade Centre Building 7 collapse even though it was never hit by an aircraft?

And why was America so unprepared when terror attack warnings had been received?

Through the internet and the media generally, allegations of complicity by the US government in the 9/11 attacks are intensifying.

We've just finished a new series called The Conspiracy Files which will start this Sunday on BBC Two at 9pm with a programme about 9/11.

We’ve talked to a number of the people who question the official version.

Dylan Avery, the 23-year-old film-maker behind the internet film Loose Change says the US government “will willingly kill its own citizens for whatever gain it seems necessary and then lie as much as they need to cover it up.”

Alex Jones, a Texan nationally syndicated radio talk show host, tells the programme “9/11 is an inside job… a false-flag terror operation.”

Jim Fetzer, former US marine and retired university professor, who helped found a coalition of academics called Scholars for 9/11 Truth repeats the Sherlock Holmes quotation “when you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

There has been an official fightback of sorts.

President Bush is on record imploring people to reject conspiracy theories: “Let us never tolerate outrageous conspiracy theories concerning the attacks of September the 11th” which he said were “malicious lies that attempt to shift the blame away from the terrorists, themselves, away from the guilty.”

The US State Department has a website to debunk conspiracy theories – not just about 9/11 but a whole range of stories circulating on the internet.

But we found that simple requests, such as asking to see the plane wreckage of flight United 93 at Shanksville, or flight American Airlines 77 at the Pentagon, were refused after months of delay by the authorities.

Yet if we had been able to film the wreckage from flight AA77 we would have had extremely strong evidence that a Boeing 757 hit the Pentagon.

Trying to prove or disprove these alternative theories is not easy.

Officials are loathe to engage, thinking that any response will only fan the flames of popular conspiracy theories, and yet no response seems to be worse still.

9/11: The Conspiracy Files travels across the United States investigating the allegations and talking to witnesses wherever possible.

Ultimately you can’t beat speaking to eyewitnesses, such as the local coroner at Shanksville, Wally Miller.

Wally Miller tells the programme how comments he made about the wreckage at Shanksville have been misquoted on the internet by people who do not “take the trouble to come here and ask me about it.”

Miller is quoted as saying, “I stopped being coroner after about 20 minutes because there were no bodies there”; yet he also said it was perfectly clear that the manner of death was a plane crash, and the point he was trying to make was that it had become a large funeral service.

The Conspiracy Files also talked to Senator Bob Graham who co-chaired the Congressional Inquiry into 9/11 which detailed the failure of the CIA and FBI to use intelligence it had received about Al Qaeda before the attacks.

Senator Graham told us there was a “collaboration of efforts among agencies and the administration to keep information out of the public’s hands.”

“Within 9/11 there are too many secrets” adding that “withholding of those secrets has eroded public confidence’ in security”.

And crucially we may not have learned about that conspiracy without the questioning of every aspect of the official version.