PaxBoy / Frank:
An estuary airport will never be built by private funds alone - way too much and way too long term for a commercial return. The public purse is going to get involved one way or another and for a nationally important infrastructure project so it should - especially if it's going to have to build a barrage anyway. Some projects just are not suited to commercial funds - railways, tunnels, airports, Olympic sites and, I suspect, nuclear power stations being among them. At some point, governments will have to recognise the fact.
Jabird: you may be right but if in 20 - 50 years time, LHR is still soldiering on:
a bit fatter, a bit busier
it will mean that London and the City will be well on the way to becoming irrelevant backwaters. That is, of course, a valid option but I don't think it's what the majority of our descendents who will have to earn a crust in an increasingly competitive world will want.