OK, 4% of our traffic will be in T4 as of Switch 1 (end March 2008), but once T4 is modified for multiple airline use (and some of the "Open Skies" boys will be starting their services in T4) its usage will start to grow, presumably back near to current levels.
Ground movements at Heathrow only just work at the best of times. Increasing the landing rate (two Gatwicks side by side) and moving inbounds and outbounds to/from two runways will very likely cause gridlock.
IIRC, the largest airport in the world (KKIA, Riyadh) is 50 sq miles, Denver is 35 sq miles. Heathrow, the "Busiest International Airport in the World", is 4.5 sq miles.
A bit before my time, but I am sure HD was correct in 1965, and it will cause big problems on the ground if it is tried in 2008.
SM