I've been to a few 40th's recently and a brilliant (but pricey) one was Bluebird Cafe on the Kings Road. They had a private room and it was really really lovely. If thats a bit too "North of the River" then the Bollingbroke on Northcote Road have a really nice room at the back, not sure if you can close it and hire it out but you used to be able to when it was the Inbreated Newt. Its right classy too!
http://www.renaissancepubs.co.uk/BB/index.htm" onclick="window.open(this.href);return false;
The Northcote has been done up recently, but I don't know if it has a private room. I also haven't tried Brasserie James which is opposite Clapham South but I have heard good things.
Failing those we have been to some parties at the Fat Deli in Balham. Its a funny place, a cross between a cafe and a bar and a restaurant, but the food is seriously good (and I mean proper London foodie good) so all the nights out I have ever had there have been a total pleasure but you won't be admiring the suede banquettes if you get my drift.
Lastly, there have been some posts here from the Steam Bar (
http://www.nappyvalleynet.com/viewtopic ... team#p1872" onclick="window.open(this.href);return false;), they seem nice so it might be worth checking out.
Good luck!!!
S