er, i've said this b4, so at the risk of boring tout:
LHR's 3rd RWY is located to the south - just next to the 2 existing ones at Gatwick.
LHR struggle to meet today's air quality targets, which have as much to do with vehicles as planes and are st to tighten in years to come. The recent fudge in seeming to lengthen the proposed 3rd rwy from the original 2500m or so for 'comuter' a/c to 3000m just strengthens my argument.
After a good fight the Govt of the day will capitulate on LHR and go for LGW instead. Don't let the 2019 embargo fool you either. It took Manchester 10 years to get a 2nd runway, so even today LGW's won't be ready till 2022 (10 + LHR public inquiry) Funny, thats, just when it will be really needed.
The Cranford Agreement will go and hence the introduction of independent mixed mode ops at LHR will offer some short term relief.
Sir George Cayley