One of London's tallest buildings (though its height depends on whether you include the mast that takes it from 175 metres to 188 metres) and certainly one of its most popular, the BT Tower (previously known as the Post Office Tower) was designed to provide support for radio, TV and telephone aerials by means of microwave signals. It was opened in 1964 and its crowning glory was the revolving restaurant, the 'Top of the Tower'. The restaurant was closed to the public in 1971 for security reasons. Until the mid-1990s the building was deemed so important to the national interest it did not appear on Ordnance Survey maps, but it’s now recognised and was listed in 2003, meaning that some of the disused aerials cannot be removed. The restaurant still revolves, though only for special functions, and the tower is closed to the public apart from very occasional openings (it was the red-hot ticket for the 2010 Open House London event). As a cultural icon it has impeccable credentials, having been utilised by ‘Doctor Who’ and ‘The Goodies’, as well as appearing on the cover of Ian McEwan’s book ‘Saturday’.