The LHR Marriott is an OK place to sleep with usable lounge, decent breakfast etc but there is NOTHING to do around that area. Stay here only if looking for a place to sleep for a next day departure. That is what I used it for and it was fine for that.

Re: Sharon and Twickenham's suggestions. Talking of good in-city rates, check to see if you have a corporate rate at St Ermins. Outstanding in-city location, generally nice hotel and on my recent stay, the rate was 179 GBP vs 132 GBP at the LHR Marriott. The other way to look at this was that MHR Marriott was way overpriced at the 132 GBP.