Confidence: It's likely to be this, but I can't be certain.
Notes: This is a species of Ctenopelmatinae, which are mostly sawfly parasitoids. A few species are nocturnal and superficially resemble ophionines. The wing venation is nicely different though, and the structure of the abdomen. There are probably three or four nocturnal Perilissus species in Britain but they are not well known.