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Hi Chris,

I got a second one on Thursday (8th) from the same location, a tiny patch of resurgent Hogweed. Under the 'scope the wing looked slightly darker, but no bristles on the end of the abdomen (sort of apparent in this image). Otherwise the same wings-folded compact stubby appearance and busy manner

If you're sure then you don't need to see specimens, the first one is

31/07/13 TQ370861 Hackney Marshes

There's about 40 native black Poplars here (P. nigra var. betulifolia) which cover everything with a thick layer of 'artificial snow', plus other hybrid Poplars, if that means anything. Perhaps the host has something to do with those species

Do you think the second fly is the same, despite the smoother abdomen? I've several more images I can crop & add if needed