I was subsequently informed that the caterpillar had been parasitised, and that it hadn't originally been that colour, so I binned the assumption about mimicry and moved on with my life.
I have made a composite image of the original images for comparison:
|'mummy' (left) and male pine flower (right) for comparison|
This morning I was amazed to find a virtually identical image where the parasitoid was identified as an Aleiodes (Braconid) wasp.
The Aleiodes lays an egg in the live moth larva and wraps the parasitised larva in a protective shell which results in the mummified appearance shown above. The Aleiodes larva feeds internally on the mummified caterpillar and pupates internally before emerging as an adult. The lifecycle of these Braconids is fascinating and complex, with some species managing to become bivoltine by parasitising different (and often unrelated) univoltine moth species which have mature larvae at different times of the year.
I suppose the Aleiodes might still gain some protection from mimicry, since there will almost certainly be hyperparasitoid wasps that target the mummies.
So. Still much research to be done, but it's nice to get even a Genus id after 13 years.