Ambitious it is, but to think it would cost less than the £13 billion it will take to build a third runway at Heathrow is to dream. London is not Hong Kong, construction costs are many times higher and the cost of a new rail corridor into London would be exhorbitant.
Of course, it should be allowed to be built if investors seek it - which means allowing for landing slots to be auctioned, and for investors to convince the big Heathrow airlines - BA, Virgin Atlantic, BMI and Lufthansa, to shift. However, the taxpayer shouldn't be involved. I look forward to a feasibility study and some accurate costs, but the UK is a very expensive place to build large infrastructure projects.
Meanwhile Heathrow remains one of the most remarkable airports in the world. Terminal 5 is perfectly pleasant as far as airports go, it HAS changed flying through Heathrow, and Virgin Atlantic's improvements at Terminal 3 give BA a run for its money. Terminal 1 is substantially improved now that BA has gone, and Star Alliance carriers are dribbling in (like Air NZ and United). Terminals 2 and 4 remain dire, but the former is to be demolished and the latter will be getting a major refurbishment. Any shift from Heathrow will see all terminal improvements there being a sunk cost, and be a massive shot in the arm for many property owners on Heathrow flightpaths, and the opposite for those immediately adjacent (because of the loss of jobs). However, London does need more airport capacity - and if it can be done commercially and efficiently beyond Heathrow, it should be.