Transfer numbers
2011 CAA Passenger Survey Data for LHR
All airlines.
Transfer traffic (assume that means overnight stays, not just connecting 'on the day', but not sure).
A random sample:
Manchester: 79%
Los Angeles: 52%
Belfast: 48%
Chicago: 42%
Paris CDG 35%
NY JFK: 35%
Amsterdam: 20%
Dubai: 20%
You can request more up to date stats from CAA.