Quis custodiet ipsos custodes? That is the question Boris Johnson, the classicist Mayor of London, found himself facing last week: who polices the police? The mayor gave a straight answer. He does – or should do. Mr Johnson forced Sir Ian Blair, commissioner of the Metropolitan police, to resign. But this episode illustrates the need to reform the confused structure of London policing.

The defining event of Sir Ian’s tenure at New Scotland Yard was the killing of Jean Charles de Menezes, a Brazilian electrician shot dead in July 2005 by armed police officers who mistook him for a suspected terrorist.