This is an ichneumonid wasp, i.e. a species of parasitoid of the family Ichneumonidae. It looks most like some species of Campopleginae, a group that lay their eggs in larvae of (mostly) Lepidoptera and emerge from the full-grown host larva. There are some species of the subfamily Cryptinae that look superficially similar and, unfortunately, it is not possible to see any of the important details on this photo. It is definitely not Diadromus, which looks very different (at least, very different in ichneumon terms...). Most people are aware of only a few ichneumonids, so the same names tend to be used over and over, whereas there are very large numbers of species - about 2,400 species of Ichneumonidae in Britain alone.