"MANFOD, give the north/south rubbish a rest will you"
Easier to be arrogant than answer some folks' legitimate questions and concerns, isn't it?
May I politely suggest Prophead that just occasionally you make it clear it is your opinion you are expressing in many cases and not absolute concrete facts.
Nobody knows exactly what will happen if and when LHR gets its 3rd runway. Some passengers may suddenly be persuaded to use an airport as a hub they have been studiously avoiding previously for whatever reason, but it's conceivable they may be happy to continue to use direct services from another airport or via a ME hub for Australia for example.
You said:
That being said LHR expansion will service more than just the north of the UK. People from the South East, Scotland, Ireland and near Europe will have access to flights from the UK's number 1 airport also.
You make it sound as if this isn't the case at present. GLA, EDI, NCL and MAN all have plenty of flights to LHR at present and as far as I know are well used. I don't dispute that if DTV or HUY for example got new services, they would likely draw in extra pax. How successful LHR would be in winning and keeping new passengers would surely depend on the transfer experience for pax and luggage and confidence that flights wouldn't be cancelled.
Comments here and elsewhere suggests that LHR's reputation in that regard, while it may have improved with new terminals, isn't always great.