Most grey flies I see basking in the sun go down as this species. Am I likely to be right?
No interactions present.
Firstly, I find the muscid-type flies quite tricky to get into the right family - although this does look like a muscid there are rather similar looking flies in Anthomyiidae, Fanniidae, Calliphoridae, Rhinophoridae and even Tachinidae. I always have to key them out to be certain of the family (partly due to my inexperience with this grop of course), and keying out relies on details of wing venation and the hairs on the body and legs, which often can't be seen in a photo.
Phaonia viarum now goes under the name Phaonia valida, but its legs are mostly (femora and tibiae) yellow, so I don't think this can be P. valida, and in fact the wing venation is wrong for genus Phaonia; the photo shows a sharp bend in vein M which Phaonia doesn't have. For a wing venation diagram see:
So I think the answer about whether it is safe to assume that "most grey flies" are this species has to be no! There are several hundred species in family Muscidae, including about 46 species in genus Phaonia alone, and a lot of other genera that look a bit similar, at least to my eyes. So I'm not at all sure what this actually is, but if I had to make a guess I would go for female Musca autumnalis.
For an idea of the bewildering diversity in family Muscidae see the Diptera.info site (which includes non-British species):
Their photo of Musca autumnalis is at:
Entomologist and biological recorder
.. and is the sun-basking behaviour too common to aid identification?
Yes it is I'm afraid!
They'll just go down as muscid flies in future then...
Lat/Lng: 49.4, -2.4
OS grid ref: S^7347