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I agree that this is in family Sepsidae, and I can't see any reason for it not to be Nemapoda, but I don't know the family well enough to confirm that. N. pectinulata is very similar, but that's supposed to be restricted to the north of Britain.
Entomologist and biological recorder
I decided the images were just about good enough to say there is no black spot on the wing - hence eliminating S. fulgens. The Nemapoda ID was from NatureSpot's Sepsidae images.
There are other Sepsids without the black spot, like Themira speces.
My Flickr photos...
Lat/Lng: 51.4337, -2.7624
OS grid ref: ST470707