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Bibio leucopterus keys out very early in Freeman & Lane (1985):
* The first couplet splits off B.venosus, which is the only British species that doesn't have a swollen fore tibia.
* The next couplet splits off B.leucopterus because it is the only British species where the spurs on the swollen fore tibia are of roughly equal lengths. The wing of the male are also milky and the pterostigma is quite brown, not black.
* Then the key splits the remaining species by the relative length of r-m and r4+5 (the V-shaped veins below the pterostigma). Bibio pomonae, marci & hortulanus all have a short r-m; while the rest have roughly equal r-m and r4+5.
* It gets a bit more complicated now - using the amount of swelling on the hind femora. Those where the femora is narrow at least 50% are all end of summer/autumn species (clavipes & lepidus); the rest have a more swollen femora
* If the antennae have 5-6 segments then you have nigriventris
* If the legs are all black you have anglicus or ferrugineus; or if they are reddish or yellow then you have "the others" (reticulatus, johannis, lanigerus, hybridus or varipes) ... but splitting them involves more features and i haven't got time to list them ;)

But yours does look like a B.leucopterus, if those spurs are equal in length ... compare to your Bibio marci if in doubt :)