Just to add my two cents.
Getting to the hotels at Heathrow never USED to be a problem when the hotels operated their own shuttles. However, a couple years ago BAA did away with the individual shuttles ( in the sake of reducing traffic congestion) and insisted on the combined hotel routes. I made the unfortunate choice ONCE of trying this. It took me nearly 90 minutes of waiting for the bus, and traveling all over before i got to my hotel NEVER AGAIN. It is either a taxi to the Sheratons or The Hilton ( via Heathrow Express to Terminal 4 and a short walk.)