The Hilton may be walking distance from Terminal 4 (most longhaul and some shorthaul BA as well as KLM and airlines handled by BAand KLM) but not for Terminals 1, 2 and 3.
Most hotels around Heathrow need transport, and don't (can't!) operate shuttle buses owing to the iniquitous policy of BAA, the airport operator, who insist on bus transfers at GBP2.50 each way per person - more expensive per mile than Concorde.
You could try Priceline. I tried the UK site a couple of weeks ago and got the Heathrow Rennaissance for GBP31 including 17.5% tax. The Renaissance is on the airport, but needs the bus ride.
The US site currently has a $10 bonus offer - try a search for the link.