A dipteran fly, probably of the family Therevidae (stiletto flies). These are similar to the Asilidae (robber flies) but feed on nectar rather than other insects. I have been unable to narrow down the identification to genus and species.
[Note added 2009-06-19: I now think that this is a member of the family Empididae or Dagger flies, possibly Empis tessallata based on the black thighs. It is also probably a female, based on the thick abdomen and the fact the eyes are well separated. Dagger flies use their proboscis to stab and kill other insects as well as to suck up nectar.]
Photo taken in Reading University grounds, Reading, UK, on 2009-04-26.