This has already been debated ad-nauseum on the following thread. This is an interesting thread, because it covers every topic of this debate, from every angle. There is something there for everyone.
http://www.pprune.org/airlines-airpo...ml#post9039270
The bottom line of my argument, is that LHR does not have the flight or ground infrastructure to be a world-hub airport. This is not about runways in the S.E., this is about an international hub airport, and LHR cannot provide that capacity. We need a 2- or 3-runway international airport-terminal, linked to a 2- or 3-runway regional terminal, which provides the regional feeder traffic to and from the international flights. And because LHR cannot accommodate lo-co carriers, I always travel to AMS for my international flights. It is quicker than travelling to LHR by train, especially since there is no overnight train to get me into LHR for 06:00. Thus LHR is not, and never can be, a world hub airport.
What we need is a 6-runway worldhub airport, with no city overflights, no noise problems, and no night ban. A true 24/7 airport. And it would be nice if it had motorway links north and south, train links to London and the west via Crossrail, TGV links to Paris and Brussels, plus HS2 links to the north and northwest. Only a Silver-Boris Thames airport can provide all of this. And in addition, the Silver-Boris Thames airport will form a part of a Thames barrage, which is long overdue and expensive in its own right.
Another runway at LHR will merely kick this can another 5 years down the road. It will not solve anything.