The Mayor of London, Boris Johnson, today announced a new initiative, London Film Day. Sunday 21 March will see 15 simultaneous world premieres at suburban cinemas across the capital from Wood Green and Wandsworth to (stretching the definition of London somewhat) Romford and Croydon. The film in question is admittedly one for families more than cinephiles: Nanny McPhee and the Big Bang, the second comedy scripted by Emma Thompson from the Nurse Matilda books. Thompson also reprises her role as the officious magical nanny.