Small black ensign fly with red eyes, looks a bit like an ant.
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You may well be right, but agreement is difficult with Sepsidae, as the features are so tiny! 10mm sounds a bit long (max wing length is listed as 4.3mm for female nitidula) and unless you took the specimen, you would not be able to eliminate other species.
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Thanks for the comments, also the size is wrong, should be 8.5 mm.
Lat/Lng: 51.329907748892, -0.75174808502197
OS grid ref: SU870597