A common flower bug, Anthocoris nemorum (Hemiptera: Anthocoridae), probably on a the leaf of a blackberry bush (Rubus sp.), though I have more frequently seen them on stinging nettles (Urtica dioica). At only 3 or 4mm long, these bugs are really quite small. The similar A. nemoralis and A. confusus tend to be a little darker and prefer to live on deciduous trees.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-06-23.