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looks like a caterpillar or sawfly larva under going a moult (ecdysis). Alternatively one of the above that has been parasitised.
This one has been parasitised, not moulting.
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Not seen this type of reaction to a parasite, we live & learn.
Sadly I don't have it, I left it in the field, I did not realise it was doomed and something interesting was going to emerge.
Thank you for all the comments.
Lat/Lng: 51.8224, -2.69
OS grid ref: SO525139