A lump of Tarmac placed where there were once fields, houses, schools, offices etc. is not a runway. It only becomes one once it is hooked up to a passenger or cargo handling facility. Unfortunately for LHR, this is the bit our ignorant politicians fail to realise. It's is but like me saying I have a gold and diamond mine in my back garden. Well I do; if you dig far enough. The thing that has not been discussed is how this marvellous lump of Tarmac will be used. Will aircraft have to cross two runways get to/from to it? Will there be enough stands to handle the extra traffic? It's a bit like buying aircraft carriers without aircraft. Until the planes arrive, it's no more than a boat.
Regarding the ATC and ground handling I truly believe there will be no problem. Every hour that LHR is open, a really excellent team of people ensure that it is operating at maximum capacity. And if they are given a third runway, they will make it work. But to get the full benefit, the operational bugs should be ironed out before anything is built. But they won't be. I have no doubt that lots of expensive children in sharp suits will be called in to provide advice and design the thing only to p!ss off smartish when their fees are paid. Their true lack of knowledge will again become apparent when the real expects have to work with their design.
Best of luck Heathrow. You may eventually get your new runway. Your real problems will start once it is finished.
PM