A digger wasp, Nysson sp. (Hymenoptera: Crabronidae). It could be either Nysson trimaculatus, Nysson spinosus or Nysson maculosus. This specimen seemed rather moribund, and hardly moved, even when I turned the leaf it was sitting on to take photos from a different angle.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK on 2009-07-19.