Actually CB, I think there could be a business case for this one (tell me if you think I'm being a class 1 idiot). Heathrow is full to overflowing; Gatwick only has a single runway and is utilised quite efficiently - therefore also nearly full up. Stansted and Luton don't have and aren't near high speed railway lines, and Luton's runway is only 2105m. Southend will only handle European traffic for the foreseeable future.
Manston, meanwhile, has one of the longest runways in the SE England and has on average just two schuduled flights each day.
Rather than chucking a massive airport in the Thames (where it will be very expensive and sea levels may rise...), it would probably be cheaper and wiser to use Manston.
You can link it to high speed railways into London and there you have competition with Heathrow, being able to take medium- and some long-haul flights which eases the strain on our already congested main London hub.
The only real issue would be noise and Ramsgate, which is a real problem.