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You could be right for all I know (!), but a book like Chinery only scratches the surface with flies (in fact, it barely does that). There are about 46 species of Phaonia in the UK, all looking pretty similar...so I admire your certainty! ;)
My Flickr photos...
ID to species (even to family and genus) from Chinery is totally impossible. You can't go past Diptera with this one.
Looks like a muscid and maybe Phaonia sp. (P.valida lacks pre-sutural acrostichals that seem to be present here).
It resembles a number of common Phaonia species but a specimen would be required for
identification purposes, even to genus, I think, in this case.
Lat/Lng: 57.2616, -5.9383
OS grid ref: NG626258