An ichneumon wasp (order Hymenoptera, family Ichneumonidae). These wasps are all parasitic: they lay their eggs in the larvae of other insects (mostly butterflies or moths) and when the eggs hatch out the young proceed to eat the host from the inside out.
When taking the above photo, I thought that the wasp was interested in the flowers. It was only later that I noticed that there was a caterpillar hidden in the flower head.
The wasp flew off a little way and landed near another caterpillar. As it approached this second caterpillar, it curved its tail under its body, as if it was going to lay an egg in it, but got disturbed by a gust of wind and flew off again.
I haven't attempted to identify the wasp; there are about 3,200 species of ichneumon wasp found in the British Isles and identifying them properly is a non-trivial exercise. The caterpillars might be the larvae of the blossom underwing moth (Orthosia miniosa), though I am not entirely sure of that. [Note added 2009-06-07: I now think that the caterpillars are Depressaria daucella.]
Photos taken in Reading University grounds, Reading, UK, on 2009-05-29.