Having lived out near MSE )in Margate) I'd say that a new airport there would be a Rather Bad Idea. By rail at the moment, it takes some 1h45min at best to get to Victoria; and road - even with the M2 links - isn't greatly faster. Coming from the west of London, you're looking at serious travel time.
In my opinion, the best potential third (or fourth or fifth or whatever) London airport would have been at Greenham Common. Thanks to the Yanks, it had most of the infrastructure already in place, including runways that could have accommodated any aircraft at MTOW. In addition, it was perfectly sited to cover the whole of southern England.
Unfortunately, the NIMBYs on the local council decided to plough the place up...
