One of the few places where I have really enjoyed eating around Heathrow is the steak restaurant at the Marriott. (There is also an upscale Italian restaurant there - looks good but I have never tried it). Given your arrival time and location I don't think that you have a lot of other options - so maybe you shouldn't aim for the top on this occasion.

Also:
* There used to be a brasserie Roux at the Sofitel, but they closed it and I haven't yet tried the replacement. (Anyone?)
* The Crowne Plaza now has a decent Indian restaurant. ("Eriki" I think it's called?)
* A short walk from the Marriott (leaving the hotel, turning right and right) there is The Pheasant. The food is OK (again, considering where you are). But I don't know what the atmosphere is like in the restaurant - I have only eaten in the pub.

And whatever you do, stay away from the Sheraton's kitchen, next to the Marriott - horrible food, even by Heathrow standards.