I photographed this fly feeding on mint flowers in the garden. With the yellow/black bands on the abdomen and boldly striped thorax, I believe it is Locust Blow Fly (Stomorhina lunata). This species is a visitor from the continent, where it is a predator of the egg pods of locusts (but ,they think, it doesn’t predate any UK grasshopper species). Not sure about its status in Gloucestershire, but although once rare in UK this fly has been turning up more frequently in recent years.