In studying Lucifer's map it would seem that the additional runway is far too far away in the northwest, especially if it is used to by smaller "commuter" traffic, where ground taxiing is relatively greater.
Ironically, the obvious place to put a 6 or 7,000 foot strip would be where T5 is slated to be, ie. west from T3 almost to the M25. Also it would obviate the need for runway crossing a la T4 and be less obstrusive to air traffic and environmentally on the ground. But where would T5 go ? LOL