www.ukbutterflies.co.uk

Seems to be the closest ID I can find - originally thought Red Admiral but I'm totally rubbish at IDs

Feeding on Nettles (if you hadn't guessed) and with another half dozen or so on different plants - at Potteric Carr (Doncaster)

Looks like Small Tortoiseshell to me, which feed gregariously and disperse in the last instar.

Painted Lady do not feed communally; the eggs are laid singly and usually on thistles.

Cheers,

- Pete

Brilliant! Thanks, Pete! Definitely looks like Small Torty despite the fact they were pretty large caterpillars!