Sacha wrote:
This seems certain to be the Maculinea arion that was suggested to me
earlier when I said one had been sighted here. Since then, the
people who saw it have been shown photographs of it and all seem to
agree this is it. I believe it was released on Dartmoor which is
about 3 miles from us so it seems very possible it would find its way
here.

This was discussed on BBC radio 4 last week:

http://news.bbc.co.uk/2/hi/science/nature/5385488.stm