Gonepteryx rhamni Brimstone
This butterfly feeds on Rhamnus cathartica (Buckthorn) and Frangula alnus (Alder Buckthorn) neither of which grow in any quantity near home so I tend to see this butterfly on my travels down south or in the limestone dales of Derbyshire. There were very few plants in flower following the severe winter of 2009/2010 so the butterfly had to make do with what was there: various Prunus species and Viola odorata (Sweet Violet).
Edge of Devil's Dyke, near Newmarket, Cambs, 9th April 2010
Added on 12th April 2010, updated April 12th 2010