While Dave B and Hiro sort out the room reservation at IC for the May F2F here's some preliminary information on hotels in the area:
Hotels can be booked through the university conference office http://www.imperialcollege-conferencelink.com/
They list hotels by walking distance from the campus and quality. They apparently arrange discount on prices.
To get to the list of hotels click the South Kensington link at the top and then select Hotel Directory.
The Campus is South Kensington.
(The closest Marriot is the Kensington one on Cromwell road.)
and
Here are some useful links on accommodation close to Imperial.
General location and nearby interests http://www.upmystreet.com/overview/?l1=SW72AZ
Hotels nearby http://www.upmystreet.com/nrs/?l1=SW72AZ&cat=1231