If that's full, I sometimes book in at the Grange Fitzrovia on Bolsover Street. The single rooms there are great, the best of all the three. Both are fairly posh, in neighbourhoods with good stuff around it, and you can get good deals off the otherwise stiff prices.
We used to stay at the Fitzrovia (before it was a Grange), but have avoided it in recent years because of accounts of deterioration. Have you been there recently, and was it OK? - DvF