Where could a theoretical fourth runway be built ?.
And isn't there an issue that if you had four runways operating then with the increase in traffic over central London plus London city , is there not a point where airspace capacity in the region is reached ?.
It keeps me in mind of that BBC docudrama back in 2003 " The day Britain stopped" , It dramatized the events after a meltdown of the transport network and the effects on over worked Atc .
Perhaps now is the time to review ALL of the British airports and how they can be used to help with easing capacity.