It is an ichneumonid wasp, possibly Ichneumon stramentor
You'll see from that discussion (2nd link) that it is difficult to be sure of an ID, at least without microscopic examination of the genitals. We have discussed the difficulty of reliable Ichnemon ID on NaturePlus before - see my posts here - http://www.nhm.ac.uk/natureplus/message/34845#34845.
Because of the difficulty of separating these similar species, at least in the field, they are often IDd as Ichneumon suspiciosus. That is not a true species, but rather an aggregate species name - covering any Ichneumon similar to I. stramentor.
Some other British Ichneumonid wasps, for reference