On the general subject of another runway in the London area:
London area airports (LHR/LGW/LCY/LTN/STN/SEN) movements:
2001: 1.075m
2011: 1.072m
In the next 10-20 years we will have airlines having to comply with carbon trading, and the UK government having to meet its Climate Change Act 2008 requirement to cut emissions of green house gas emissions by 80% below 1990 levels by 2050.
In addition we have seen a shift towards larger aircraft, with turboprops and E135/145 types now rare at Heathrow while A380 movements are increasing.
On top of that I can't see any prospect of fuel prices doing anything other than a continuous upward trend.
There's no need for additional runways.
NS