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Thanks Andre. I'm sure you are right that a photo alone can't be definitive. I got some extra advice from David Clements, though, who organises the British Conopid recording scheme and he tells me it looks very like a male Leopoldius signatus. He says the near completely yellow scutellum and apparent lack of a long proboscis look right for Leopoldius and the flight date, location (on ivy leaves near ivy flowers) and what is visible of the abdominal pattern are all strongly suggestive of L. signatus. It's locally well distributed in the south of the UK and is a parasitoid of common wasps (we have many....). The other UK Leopoldius, L. brevirostris is much rarer and mostly associated with semi natural woodland. So I guess it's "probably" L. signatus.

Nick