I'd suggest G. himalayense from the leaf shape (and flower characteristics)... (although it seems to me that leaf shapes can be somewhat variable). See Peter Yeo's Hardy Geraniums (2nd. Ed.) for an excellent reference that would allow you to key it out.
I think we've also used the European Garden Flora in the past to help us identify geraniums (and, if I recall correctly, it _really_ helps to have the plant in hand).
