Soldier Fly - Stratiomyidae

This extremely pretty metallic fly is a Soldier Fly aka Broad Centurion. There have been a few around the allotment this year - glistening like a jewel in the sunlight

From what I've seen online it would appear that this is a female Chloromyia formosa - the males have a bronse abdomen rather than this pretty blue one.
They are fond of wild parsnip (and maybe parsnips which have gone to seed on the allotment) along with hogweed. This one was resting on our broad beans but shouldn't be a problem on the allotment. The larvae develop in decomposing matter so probably came from our compost bin.