A capsid bug, probably Capsis ater (Hemiptera: Miridae). In the summer I often find these in long the grass climbing up and down the stalks. The swelling at the outer end of the 2nd antennal segment is a characteristic of this species. This specimen shows more orange than the one in my earlier photo here.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-06-27.