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It's always useful to measure the size (length) of these flies - Bibio marci is the largest of the all-black ones, but there are similar smaller species. Difficult to judge from the photo alone.
I like the in-flight shot!
Entomologist and biological recorder
whoops forgot to add size was about 15mms length.
... in which case I think it is safe to call it Bibio marci!
Lat/Lng: 51.4, -2.7
OS grid ref: ST5469