I have Heriades Truncorum using the 'bee posts' that I have provided. However, not satisfied with the accommodation I have provided, they are tunneling into the sandy mortar of my garage wall.This morning I noticed what I presume are flies, two at a time, shadowing the bees in flight and then standing guard outside the tunnel. They seem malevolent and I think one of the two will enter the tunnel, possibly when the bee departs.
I attach a picture of one of these. It is about the size of a housefly.
I can't see any golden hairs and it doesn't look much like a cluster fly to me. That said, I don't know what it is. Unless someone else comes up with an ID it might be best to go on to diptera.info and search through their picture gallery, maybe starting with the Muscidae (?)