A soldier fly, species Chloromyia formosa (Diptera: Stratiomyidae). In this species the females have a metallic blue-green abdomen while in the males it is gold. This difference is most noticable in flight when the abdomen is not covered by the wings. Chloromyia speciosa is similar but the lower parts of the hind legs are yellowish (see here).
Photos taken in the long grass in Whiteknights Park, Reading University grounds, Reading, UK, back on 2009-06-04.