The Original 1947/8 plan was for 9 runways, 6 in a "Star of David" on the existing site . 6 of these were there at one time or another until Central Area cut across 3 of them. The other 3 were to be a triangle north of the A4 and the line of the M4 was accordingly bent to the north to accomodate this. Its alignment as the intended airport boundary is the only remaining tangible evidence of the far sighted scheme. Sipson and Harmondsworth, such as they were then, were to be largely demolished. This visionary plan was abandoned circa 1953/4 and the threat to the 2 villages lifted.
The capital cost of moving LHR somewhere else is probably too high for UK Plc to cope with. It has too many other political priorities, - eg schools, hospitals etc etc.
The new plan for the 3rd runway and 6th terminal will have short term environmental benefits in cutting stacking, taxiway queues etc and longer term is essential if LHR is not to wither on the vine. There is also a need for improved rail links including the linking of the T5 Heathrow Express and Cross rails back to the nearby western mainline as well as the occasionally spoken of scheme to provide a loop to the Feltham line and back to Waterloo. The western line to Reading would provide direct rail links to the west, north , Wales and the south west and it's incredible that a link to it is not even a blip on the horizon.