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David, thanks for your response. As to your question of the caterpillars size, from memory (not always so reliable), about 4 or 5cm.
I have made a number of failed attempts to find this species and so would be pleased if you manage to come up with an identification.
Lat/Lng: 50.404934498871, -4.102578163147
OS grid ref: SX506582
The field has woodland on three sides and is bordered by bracken, bramble, small and medium plants as well as elder. There is also an island of small trees growing up through rosebay willow-herb.