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Like an early Peacock caterpillar, before the white spotting appears.
But that would be feeding on Nettle (possibly Hop), which doesn't seem to be the case here.
It was on a lesser burdock leaf, from memory. Could that still be a peacock?
But it might have been knocked off a nearly nettle?
My old book lists several species that feed on Burdock, but none are a good match visually - in the book at least.
But one photo on the net -
shows a caterpillar of a related species, and there is a similarity.
See what you think.
That looks like a possibility! Thank you for your help!
Lat/Lng: 51.8044, 0.4113
OS grid ref: TL663145