The Open House weekend, when buildings across London open their doors to visitors, gets bigger every year: the most recent, its 23rd, featured more than 800 buildings and 2600 architect-led tours. Part of the pleasure lies in discovering so much of the city that is hidden from you: 10 Downing Street, the Cheesegrater, banks, halls and bell-towers; it's like walking onto a movie set, or into another life. It also indulges your nosiness about how other people live (and where): who knew there’s a three-bedroom flat at the top of St Pancras Station's clock tower, or that Lewisham has a whole cul-de-sac built on stilts?