Paxboy, as much as I understand the reasoning, Thames Gateway would, once Crossrail is complete, make much more sense... That's why Crossrail was demanded in the first place (i.e. shift lots of SLF from one end of London to the other without bothering with UG/Bus/Taxi/Car/OG). Thanks to parliamentary inaction and the like, Crossrail's been sitting around gathering dust for at least a decade (well, the bill was finally passed, but that still doesn't mean they'll jump into action anytime soon).
This eternal dithering is going to cost London a lot more than 12 billion for a new runway at LHR, a proposed fourth runway around 2020 (for probably another 25 billion if the 12 billion is anything to go by), or a brand new airport for around the 12 billion mark.
The Economist has a great piece on this... I don't have my copy on me, but if you go to a newsagent that stocks it, you can't miss it... it has a huge headline on this on the front cover. Perhaps I should scan it in and post it.
S.