You stay at the terminal linked by an indoor bridge. This is first & foremost a lovely hotel which, secondly, is conveniently located for travel from Heathrow...instead of the other way round. You get away from all that is pre-travel and relax with the services fo a very good restaurant, a lounge bar and a harmonious tea & coffee lounge to name but a few of the facilities. As well as being a great stay I also use the hotel for those extra meetings I squeeze into the beginning or end of my business travel itineray to the UK. It's the perfect place for these.
Also. remeber that the Heathrow Express operates free for inter-terminal transfers so even if you're leaving/arriving form another terminal, you can access this hotel from the lift at the HE train station under terminal 5.