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Thanks for the comments. So it looks to be correct that it is a Polietes species.
I decided to use the (old) Muscid key, and there are only 4 species in the UK, which helps a lot!

It isn't P.lardarius, and it isn't steini (eyes almost bare, and it goes for horse dung only, but mine is definitely dung from rare cattle), and it isn't albolineatus, which has slightly different venation, and a single broad dust stripe on the thorax. So by elimination alone it is Polietes hirticrus. And it also fits with the key, in having mid tibia with postero-ventral bristles, densely hairy eyes, and a lower squama conspicuously yellow.

(I often find that yellow appears more orange in photos - it depends on the degree of illumination).

Although I am not an expert, I believe this is correct. It is an unusual species, with only 25 records, and I can find absolutely no images on the web, or on this site.