Two Heathrow hotels I have used that offer consistently stonking rates would be -
Heathrow Sheraton (NB NOT the Sheraton Skyline which is more expensive)
and
Heathrow Novotel
The Novtel is a bit further away and requires a fare on the Heathrow Hoppa bus if you have no car. Heathrow Sheraton you can do by local bus if need be.
I found bookable links to both on idDeals.com where small membership fee required, but you will save more than that with one night.