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Hi Stuart
I have just been identifying specimens of Machimus atricapillus and M. cingulatus. It will be difficult to make a certain ID from a photograph, but I would say these are very probably M atricapillus, as M cingulatus is a quite brown colour. The flies in the photo appear very grey. In my experience M atricapillus is by far the most common UK member of the genus, so I doubt it's one of the others, which have far less orange on the posterior of the front and mid tibia. These flies have quite extensive orange on the tibia, again indicating M atricapillus or M cingulatus. I am assuming that these are UK flies?

(I haven't got used to calling them Tolmerus yet:D).