A small black sawfly, probably a Tenthredinid Hymenoptera: Tenthredinidae). It looks similar to some species of Dolerus but I don't feel confident in picking out which species. [Note added 2010-11-08: The head looks too shiny for Dolerus, which are quite furry about the head. It is most likely some other genus.]
Photos taken in the Wilderness, Whiteknights Park, Reading University grounds, Reading, UK, on 2010-04-11.