I initially thought this was a Calliphorid and it was only a week later when I got round to trying to identify the species did I realize that it was more likely to be a Tachinid, probably Pales pavida (Diptera: Tachinidae). [Note added 2011-09-19: This identification was confirmed by Theo Zeegers here).]
Note hairs on the lower halves of the eyes (yes, hairs actually on the eyes!).
Photos taken in Whiteknights Park, Reading, UK, on 2010-07-18.