A dagger fly (Diptera: Empididae), possibly Empis digramma [Note added 2009-09-05: I now think these are both Empis Lutea based on photos here, here, here, here and here.], feeding on hogweed (Heracleum sphondylium). The tubular shape of the abdomen suggests that this is a male.
Photo taken in Whiteknights Park, Reading University grounds, Reading UK, on 2009-07-14.
The following photo was taken a couple of days later (2009-07-16). Here the tapered abdomen suggests a female.