One I remembered last night. The Harp, Covent Garden

http://www.beerintheevening.com/pubs.../Covent_Garden

Excellent selection of guest ales (plus cider and perry). It isn't big, but standing outside on warm summers evening watching the world go by with a pint of ale is very civil.

If you are with friends, and fancy an evening out with a difference have a look at this

http://www.watercressline.co.uk/rat.htm

Easy 45min train ride from Waterloo, perfectly do-able in an evening.