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I took many photos of a small fly (4-5mm) moving about on ground debris in an area of lowland bog.
It is certainly a Sphaeroceridae. Using the key to Lesser Dung Flies by Brian Pitkin, I see it is a Copromyzinae (female).
Then I get into difficulties, because I cannot be sure if there is more than one row of postocular setae.
I think there is more than one row.
I have some bristles and other features which are difficult to be sure about in field photos. So I then started comparing illustrations, rather than using the key. I have concluded that some illustrations of the head side view are not reliable in places, because they show the shapes do not always match that from live specimens, or freshly dead ones. It seems to me that only the top-down view is reliable.
I am also puzzled by the mention of a length for Crumomyia nitida, when no other species has a length. Mine happens to fit in that range, but it doesn't look quite like nitida because that seems to have all black legs, while mine has shiny black femurs, and the tibae and tarsi are paler.
Can I conclude anything? Is an identification possible to species level?

Seen in Cumbria, UK.
Thanks.
Nigel.