The Small Tortoiseshell
Last year Peter Martin wrote a fascinating series of posts on a number of butterflies that can be seen at Fleet Pond. You may wish to reread them:
April: The Brimstone
May: The Orange Tip
June: The Small Heath
July: The Small Tortoiseshell
August: The Gatekeeper
September: The Red Admiral
The Orange Tip
Picture credits: Wikipedia, full details are on the relevant posts above.