If you're looking for an alternative to the Park Plazas, I've stayed in a couple of the Radisson Edwardians, all very nice:
Sussex - Behind Oxford Street, very cramped
Berkshire - Behind Oxford Street, slightly less cramped
Bloomsbury - still very central, right by Tottenham Ct Road
New Providence Wharf (Canary Wharf) - outside the centre so very spacious
Really taken with the NPW one - very modern (and that was in early 2009) - just slightly out of the way