What we have seen today, re the snow, suggests that having all your eggs in one basket is not sensible. We have the M25 around London a major traffic route being totally blocked most of the day, so adding further lanes would just add greater concentration to the chaos.
Likewise having a further R/W in the same location as the existing R/Ws at LHR would just add to the chaos when the w/x turns bad. Military men always suggest that assets should be spaced out to avoid collateral damage, I believe the same thoughts should be applied to our major airports.
Most of the delays in the North of England were caused by the London hub being out of action. Aviation always works on the mantra of redundancy, clearly fortress LHR does not follow that idea, alternative routes must lead to a more robust transport network.