How do the police investigate themselves?

How should Britain's biggest police force, the Metropolitan police, tackle corruption by its own officers?

This year, fresh doubts have emerged as to whether the force has ever dealt with it properly. They were sparked by a review by the barrister Mark Ellison who suggested the detective who investigated Stephen Lawrence's murder had acted corruptly and that documents had been shredded.

The Ellison review also added to concerns about the conduct of undercover officers in the Met. The BBC's home affairs correspondent, Danny Shaw has been given unprecedented access to the Scotland Yard department which carries out internal investigations.