There's just one problem with a 3rd runway at LHR. Let us assume that it is railroaded through (pun intended!) and opens on 1st April 2012.
BAA plc will immediately over sell it by oferring more slots than they can handle at the terminals, taxiways and stands (as they have done with the two primaries). So the capacity would have been expanded but the
capacity issue would remian the same. If they then built a 4th, the same would happen.
It would happen because there is no policy on air traffic in the UK and has not been a policy since the airports and BA were sold off. The vested interests are not too great and can weild too big a financial imposition to change it.