I see these flies in large numbers every spring. They fly slowly with hind legs dangling.
No interactions present.
You definitely have "a" St Mark's fly, in family Bibionidae, and it could well be "the" St Mark's fly, Bibio marci, but I'm just a bit hesitant without knowing the size. It's difficult to judge the size from the photos, but at first glance it looked a bit small for B. marci to me.
However, the general shape, colour and hairiness looks correct for B. marci, so I'm not going to dispute it!, but it would be a useful double-check next time you see one to get a measurement of its length (from front of head to tip of abdomen, or atlernatively the length of one of the wings).
Entomologist and biological recorder
Lat/Lng: 52.6, 0.4
OS grid ref: TF6511