Some very interesting comments on both the LGW/LHR threads. If this was a free market solution rather than ultimately a political one, then you'd probably build more capacity at both, LHR is for all intents and purposes full, whilst over at LGW with just a single runway, the busiest single runway in the world, there is still some quieter periods.
The politics will be key of course, the politics probably favour LGW, there are no marginal seats around LGW for the Conservatives, Boris will apply huge pressure to avoid a yes for LHR ditto Zac, who is now standing as London Mayor, of course giving the green light to LGW will not solve LHR's capacity issues but will provide a quicker and less expensive capacity jump to the South East.
Having read the Gatwick submission in full ( a boring standby day !!) it has much merit, the simplicity of the layout is key and the last thing the M25 needs is another major construction project of this size in West London, no underground public access under airport facilities, one motorway access point, centralised car parking are but a few of the public benefits, sadly it doesn't solve the LHR question