Meanwhile, I've had this turn up at a recent moth trapping session (see photo). It's been identified by an expert at the Natural History Museum as Opheltes glaucopterus, and it's big. As in close to an inch long, which by Ichneumon Wasp standards in the UK, is pretty big - most species in the UK fauna have a body length under 10mm, and some are as small as 3.5 mm long. The big ones, as a consequence, tend to be noticeable among entomologists, even those who specialise in other insect groups, because the big ones stand out, and this one stood out because it was even bigger than the usual Ophion species that turn up at moth traps.
Anyway, I sent the photo and the record details off to my local biological records centre, and the only other record they have of this species in my area dates back to, wait for it, 1947. So apparently I've found the first individual of this species in my area for nearly 72 years.
This beast makes its living as a parasitoid on the larvae of large Cimbicid sawflies.