If this is female, is it laying eggs in host fly?
No interactions present.
They are predatory upon other diptera. It looks as though the victim here is a Downlooker Snipefly...quite a big prey item. According to Collyer and Hammond, they seize the prey by the thorax, insert the proboscis and extract the juices from the head first, then the abdomen.
My Flickr photos...
Lat/Lng: 58.4759, -6.21412
OS grid ref: NB544618