london city is close to heathrow and that works - i'm sure with some re-organisation, northolt could get more flights in an out, lots more. the northolt runway could be extended at the western end if you buried the A40 in a tunnel, or the best solution would be a completely new, and very long, parallel runway on the green land south of the A40 (no need to flatten any houses). there is already a major motorway going right through the middle of the site and a railway line at the eastern end – it could scarcely be better...