A third satelite (5D) for LHR-5 is very likely, as IB are moving into LHR-5 and several BA services really need to brought accross from LHR-3 ASAP. Otherwise BA remains in a split terminal operation situation (as was the case when it was based in LHR-1/LHR-4) that the LHR-5 development was intended to resolve.
It would be sensible to extend the underground transit to LHR-3 to facilitate easy transfers between Oneworld alliance carriers, and indeed on to the new LHR-1/2 as well to improve most transfers. A parallel landside transit should be on the wish-list as well with an additional stop at the central bus, rail and tube station!!
Meanwhile, back in the real world.....
Without extra runway capacity this terminal development would still be needed as there will be a trend towards the use of larger aircraft to compensate for the lack of available slots.
Of course if the "stupid nimbys/putney woman get their own way" (to quote
adfly) it could be a massive own-goal. At present, there is a half-day respite from noise for those under the flightpath. Alternating runways between takeoffs and landings with a changeover at 1500 ensures this.
Without runway expansion there will be unstoppable pressure for mixed-mode, it has already started on a "limited" and "experimental" basis. Apparently mixed mode could raise the 480,000 annual movements to 540,000.
With runway expansion, annual movements go up to 702,000
AFAIK. However those
currently under the flightpath would not be affected.
The truth is that, ironically, these residents could
actually benefit from runway expansion, because they can then ensure that alternation is maintained on the existing runways as part of the conditions for allowing the expansion. This is where their efforts should be directed.