Black Muscid flies

For the past few months I have been using Fonseca's key (Handbook for the Identification of British Insects - Diptera Cyclorrhapha Calyptrata Section (b) Muscidae by E. C. M d'Assis Fonseca, 1968) to identify Muscid flies I have collected.  I was having trouble with some small black female flies which kept running to Dialytina atriceps (=Phaonia atriceps) but which definitely had only 2 posterior bristles on their mid tibae instead of the required 3. Well I think I have just solved the problem: they are Phaonia halterata. Instead of keying out before you enter the main Phaonia subkey they run right to the last couplet of the that subkey.  I now have to go back and recheck all the 20 flies I have already assigned to atriceps.  Using keys takes quite a lot of gettting used to.