Well summarised Trash 'n' Navs. I've had 40 minut taxi/idle out and 30 mins taxi/idle in on numerous occaisions and that emiision pollution is all at ground level.
I would place a limit on the total movements so as to spread the load across the three as we currently (probably) have about 2.5 runway's worth of traffic. Which is why we actually need four runways to absorb the 'over capacity' we currently have.
The fact that none of this will happen, and the reasons for that, are well understood and (repeating myself) LHR was long doomed to be in the second tier of European hubs - as it already is.