Over the years I must have stopped in almost every hotel at LHR. I never book more than 2 days in advance and just take what late deal is available. There are so many hotels at the airport than there are always deals to be had and if I'm only going to be there one night (and often only 6 hours) then I'm not too fussed about which one it is. My favourite though is probably the Crowne Plaza even if it is further out than others.