Further to Peachy Steve's find of a colony of Parsnip Moth larvae at Sowerby Bridge I took half a dozen home to rear through. Just one of them managed to reach the pupal stage, the other five were all parasitised by a species of Chalcid wasp. At first I thought they had all been "stung" multiple times but in fact that was far from the case. I sent a couple of photos off to wasp expert Dick Askew and here is his reply:
The parasitoid larvae in the Parsnip Moth caterpillar look to me like those of Copidosoma. This is in family Encyrtidae (Chalcidoidea) and they are polyembryonic, a brood of very many individuals all of the same sex developing by division of a single egg. I do not know whether or not yours will overwinter, but if they do it will probably be as fully grown larvae (ie. much as they are now). Best to keep them in a shed or outhouse."
Above, you can clearly see the dozens of grubs through the larval skin - nice!
Below, one I inspected earlier to see why the larva wasn't pupating as expected.