A chalcid wasp, possibly member of the family Torymidae (order Hymenoptera), but I'm not entirely sure of that. [Note added 2009-09-14: I now think these are Ormyrus sp. (Hymenoptera: Ormyridae).] It is also probably a male because it lacks the long ovipositor that most female chalcid wasps seem to possess. Like the closely related ichneumon wasps, chalcid wasps are mostly parasitic on the larvae or eggs of other insects.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-08-22.
A female of the same species showing the ovipositor. In the past week I have seen one of these twice in almost exactly the same place, but I only managed to get one, rather blurry, photo. The problem was that, whenever I went near it, the insect would drop off the leaf into the long grass. Presumably this is a defence mechanism against predators, but it also makes things very awkward for photographers.
Photo taken in Reading University grounds on 2009-08-30.