Peter,
I've always thought that if Heathrow is to have a third runway then its has to be at Northolt. You would need to realign the existing runway so that it is parrallel with the the two at Heathrow. This could be done within the existing perimeter.
That is a more interesting suggestion. But I think the locals would smell a rat, and they'd see that as a brand new runway, which it effectively would be.
Afaik, aircraft like the 320neo claim a reduction in fuel burn around 15% compared to predecessors, but is there a corresponding reduction in noise? I know the Q400 claims a noise footprint of just 5% of a 732, but they've all long gone, and you'd need double the movements for the same volume of pax. Not what LHR needs.
As for airside-airside, this wouldn't be anything like as easy as at FRA, where the link is above ground, and presumably designed into the terminal from the start.
As LHR has 3 main terminal areas (anyone got a plan for where T6 would go?) - that would be one hell of a challenge as you'd be dealing with a complex quiltwork of tunnels. And I think it would be equally difficult to retrofit the existing rail and tube stations. But probably still cheaper than BI
