A late-season bluebottle fly, Calliphora sp. (Diptera: Calliphoridae), a male because the gap between its eyes is relatively small (compare wirth the female here). This specimen was only about the size of a house-fly, a bit smaller than those I remember from the summer. In the last few weeks I have also seen what I think were small Pollenia.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-11-23.