On a walk round the lakes this evening, Zoe spotted these on some stinging nettles (Urtica dioica). She guessed that they probably were the caterpillars of some colourful butterfly, and she was right: they are caterpillars of the peacock butterfly (Inachis io).
There were also some smaller ones of the same species on a nettle plant a bit further on:
And, on another nettle plant, there were even smaller ones enmeshed in what I presume was a protective web:
Photos taken in Reading University grounds, Reading, UK, on 2009-06-09.
This species, Inachis io, is also known by the synomyn Nymphalis io. For a photo of an adult peacock butterfly see here.