It is a male Cheilosia albitarsis agg. (Hoverfly). The larvae of this species feed on Buttercup roots and the adults are almost always seen visiting Buttercup flowers. The species was split 2000 in to C. albitarsis and C. rannunculi and the best way to tell the difference is by examining the male genitalia, but there is also a slight differennce in the shape of the front foot (5th tarsal segment) in the male - but unfortunately that is out of focus here. The females of this pair cannot be reliably separated. This is a male because the eyes meet on top of the head, but we can only say "agg." - its one or the other.