#2
According to Stubbs and Drake (
British Soldierflies and their Allies) the choice is is either
B. fuscipes or
B. geniculata:
B.fuscipes Abdomen black, legs extensively dark, antennae short and swollen at base of flagellum. Local
B.geniculata Abdomen black, legs black with orange knees, antennae longer and more slender than in fuscipes. Widespread.
#3
Thanks for your help Tristram.
Looking at images of fuscipes shows pale areas of the legs which are not present in my specimen. I think it seems most likely then to be geniculata.