So after much delay and prevarication to avoid upset at the last election for so many Tory supporting areas, the airports commission has come out in favour of a third runway to be built on the north side of Heathrow:
http://www.bbc.co.uk/news/business-33340565
Inevitably many local campaigners, MPs, current London Mayor Boris Johnson, and others are all saying it's wrong. But many others, especially the business lobby support Heathrow expansion.
In my view you wouldn't build a new airport at Heathrow today, but the world was a different place when it started as a few tents in the 1950s. London Suburbs have expanded, and as Heathrow grew, so did the housing and other infrastructure that both supports and relies on it.
Heathrow is arguably better connected by road, and rail than the perceived alternative of Gatwick. But Gatwick expansion would be cheaper, and probably cause less disturbance to many. Expansion of either airports still warrants improved local transport.
Gatwick is the worlds busiest single runway airport. It and Heathrow are very close to capacity. In reality both airports are in need of expansion for different reasons. Indeed I think both airports should have an added runway, Heathrow so the much lauded hub status can be maintained, but Gatwick so that some of the Heathrow disturbance and capacity can be alleviated and spread around.