A weevil, probably either Phyllobius sp. or Polydrusus sp (Coleoptera: Curculionidae) on an oak leaf. These two genera are rather difficult to distinguish, as shown by these pictures, and by this post at Yahoo! Answers. In particular, the advice about hooks on the upper part of the legs, given on this page at the Garden Safari seems to be unreliable.
Also, I do not think that it can be Pachyrinus lethierryi (in spite of this photo at Wikipedia) because the latter should have a broad snout and be associated with juniper bushes and cypress trees (the Wikipedia photo may have been misidentified).
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-09-05.