Interesting that most of the 'celeb' sightings in Marriotts so far have been sports figures.
I saw quite a few of the Oscar/BAFTA nominees at Grosvenor House ballroom 2 separate years (George Clooney, Charlize Theron, etc), but that's because they were there for the dinner following the BAFTA Awards. Kevin Spacey was in the Red Bar at Grosvenor House following the Olivier Awards.
Cheers.