Wasp

Originally I had this down as an Ichneumon wasp but I now think it is a wasp of the genus Gasteruption  (Hymenoptera: Gasteruptiidae).   The narrow waist, the long thin black and red abdomen raised above the folded wings, the long ovipositor (in females), and the swollen hind tibiae, are all characteristic of this genus.   For many more photos of members of this genus, see this page at Alain Ramel's Les Insectes site (starting about a fifth of the way down).

Photo taken in the field below Chazey Wood, near Caversham, UK, on 2011-07-11.