A long-legged tachinid fly, most probably Dexia vacua (Diptera: Tachinidae), based mainly on comparison with photos here. [Note added 2010-07-18: I now think it is Dexiosoma caninum (Diptera: Tachinidae).]
I first saw one of these flies back on July 16 only a few metres away from where these photos were taken. In the intervening weeks, I repeatedly went out of my way to try to get more photos of these flies because I wasn't sure whether they were tachnid or rhagionid flies. I have managed to see them on 2 or 3 further occasions but only in that small group of bushes, never anywhere else, and I have never seen more than one of them at a time.
Photos taken in Whiteknights Park, Reading University grounds on 2009-08-12.