Two black and green sawflies mating. Probably Tenthredo mesomelas (order Hymenoptera, family Tenthredinidae). Distinguishing features are the light green spot on the thorax and the black leading edges to the wings. The similar Rhodogaster viridis has green or yellow leading edges to its wings. (Ref: M. Chinery, Complete British Insects, Collins, 2005, p 324). Both of these species seem to show quite a lot of variability in the ratio of black to green on the upper parts of the body.
Photo taken in Reading University grounds, Reading, UK, on 2009-05-22.