Thanks for reviews, just booked yesterday for a December stay (on points, as the room rate w/o tax was already approaching £200). So far Plaza on the River is my absolute favourite, which however, is a bit further out and lacks sauna/pool.

I'm a bit wary about the sheer size of the Westminster one, and it doesn't sound like a very warm place (tourist bunker full of staff that could be more positive?!).

Edit: Was in touch with hotel by email - upgrades at this hotel are limited to the most desirable rooms of the BOOKED room category. So this seems one of the few Club Carlson properties that don't provide category upgrades.