Depending on your budget I lived for a while (for work) in the Travel Inn Hampstead (actually in Belsize Park) which was a fantastic hotel and very convenient but may be more than you want to spend.
http://www.premiertravelinn.com/pti/hotelInfor mation.do?hotelId=25094
I also stayed in the Maranton B&B in Earls Court, again very convenient and nice rooms, no lift though if anyone has a problem with stairs.
I'd suggest ringing them and asking if they can do you a deal, they did for me.
http://www.marantonhotel.com/
Be careful of some of the really cheap ones in Earls Court and the like though as there are some horrors!!!
It may be worth adding up what you could be spending in train fares being further out to give youself a bit more of a budget.