Well, I've pulled the trigger saving £150 on a 3 night stay at the Xenia vs the Kensington Marriott, though looking at the pictures of the petit double lighthumor was upgraded to I decided against the 9sq metre petit single and instead plumped for that double at £8 extra per night, as looking at lighthumor post I think this hotels upgrade policy is one room category only, and I don't fancy that version of a double room much. Hence booking that double should see me upgraded into at least a decently sized classic - at least that's the strategy! I'm there in 2 weeks and will update you how it goes!