Great private dining venue for 25 guests in London?

[champagnesupernova]

Throwing a surprise birthday for my dad
We had a private black tie dinner at The Ritz which was a bit OTT for my mum but lovely nevertheless and she wants to "return the favour" - something posh, with lots of fuss for him
But where?
Budget up to about £100 per head and if we can get away sans room hire so much the better

Square Meal has a good section on best private rooms for 10-30 guests here; I have always found it fairly reliable.
Most are good choices (don't touch Julie's with a barge pole, I went to a hen once there and the food was just appalling) IMO but Maze, The Square and Pied a Terre would definitely get my vote for most impressive, although there are some other great choices in there too.

mosimann's is pretty good