Netelia inedita

Strictly speaking this species ID is a "probable", but since that ID was provided by one of the foremost experts on the subject in the UK, it's a lot more confident than had it been me offering that "probable" ID.

Netelia are a sub-species of Ichneumon, a parasitoid of Geometrid larvae, particularly the genus Xanthorhoe.

At first (and second) glance they are very similar to Ophion wasps, which is what I first identified this specimen to be, but under the microscope, there are numerous differences.

Like the distantly related Ophioninae and some other groups of orange Ichneumonidae and Braconidae, Netelia are largely nocturnal.

All Netelia are koinobiont ectoparasitoids, which means their hosts continue to grow and develop even after they have been parasited. Once the host larva host has prepared a pupation site, the parasitoid rapidly consumes it and spins its own black cocoon.