When it is dark to the west of London you can see a queue of plane lights flying in one after the other, after already being stacked for ages waiting to join this queue. Apparently the stacking system is often full so they would have to be stacked elsewhere, causing yet more areas to be disturbed.
To me it seems crazy to fly even more planes over such a heavily populated area. It isn't just a matter of the additional noise but also the additional risk of a major disaster due to a plane crash or other problem.
I believe that an airport to the east of London would be more sensible - plenty of empty brownfield space there, as Boris has suggested - and I understand that flight paths could be organised to disturb far fewer people. Or it could even be built on a man-made island offshore as other international cities have done.
Interesting to note than many people who fly into Heathrow are just in transit to other places and never leave the airport. Why not direct these people to airports well away from our busy centralised sites?
DH used to be a civil engineer and says that the proposed re-routing of the M25 (possibly through a tunnel) would cause really major disruption to the M4 and M25 for a very long time. And the M4 to Heathrow is already said to be the busiest stretch of motorway in Europe.