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I believe this is a Lauxaniidae.
I tried the key for British Lauxaniidae, but since I don't have good photos of some features (eg bristles on the ocelli) it's difficult.
So I tried an alternative strategy - looking through the diptera gallery. There are some quite similar, but they were all different enough on certain features to reject. So I am left with nothing.
So I think it's down to someone recognising this one. It's not an oddity because i have come across a more-or-less identical species a year or so ago, and mis-identified it as Sapromyza albiceps.

Found in Cumbria, UK. End of May.

Nigel