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I believe this is a Muscidae. It has been suggested that the scutellum is deformed - it appears to form a half-cup shape rather than being relatively flat, or tilted.

I initially rejected Tachinidae because the wing venation is wrong. The images as a whole are probably not clear enough to use a key, so I hope someone can recognise the fly.

Size about 5 mm. Seen 04/04/2020, south Cumbria.
More images to follow.

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A closer view of the front end.

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A slightly different view of the front end.

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A dorsal view.

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Thricops semicinereus?

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I don't recall ever having seen a male with completely orange abdomen...

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I have just done a trawl through Diptera.info images, and agree with Tony about that point - no males with an all-orange abdomen.
I also think that mine has a more dished face than any I can see anywhere.

Nigel.

By the way, thanks for the good starting suggestion, Paul!

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Hydrotaea meridionalis? That white spot by the antennae made me think Azeliini, and H. meridionalis is another bicolored species.

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Dear friends, I also tried to solve this mystery in the morning. Now I'm back, but still without answer so far.

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An interesting suggestion, John. But I can't find any images of a male, but just females.
I am pretty sure that the white thing at the top of the antennae is a clump of pollen, similar to some of the larger blobs on the thorax.

I see there is a thread at https://diptera.info/forum/viewthread.php?thread_id=45541
I don't think there was a conclusion to this post! ... though it was a female, not a male.

Nigel

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H. meridionalis looks similar to H. borussica https://diptera.info/photogallery.php?photo_id=3309
John's idea was good, but I doubt that it is answer. I have a lot of material with me (for co-quarantine: me+Diptera). In H. irritance group f3 and t3 should be strongly curved.
Muscid on these imags looks as Lophosceles hians with dark legs https://diptera.info/photogallery.php?photo_id=6268
Sorry, I have no idea.

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Hi everyone,

the white spot is similar to Hydrotaea pellucens. borussica and meridionalis males have a very noticeable brush of feathery bristles sticking out of the middle of the hind tibia. I think that Hydrotaea pellucens is a better match. The photos posted are ridiculously small for debating features, so the author of the post will have to examine larger photos for comparison to Hydrotaea pellucens.

Best wishes,
John

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I have trouble seeing this as a Hydrotaea - the legs seem to me to be too simple and lack the modifications typical of that genus.

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Looks like Fanniidae or Anthomyiidae, not Muscidae.

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It appears to have 4 postsutural dorsocentrals, Sc bent as usual in Muscoidea, A1 ending halfway to wing margin, and A2 not curved enough to meet the extension of A1.

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I have looked at the image of Lophosceles hians, and (apart from the yellow legs!) looks very similar, except that mine has at least 2 prominent spines in the middle of the hind tibia, but the photo specimen does not have any in the middle. So I suggest it's probably not that species.

I have looked at my images of the legs in case this is useful, and it's difficult to see properly in poor light (in the shade of a huge Picea pungens!), so this is the best photo, post-processed.