Dancing flies, shame i could not do video of them as the dance went on for several minutes back and forward, side to side and with various other movements.
No interactions present.
Looks like a good match for C. flava, but I'm not 100% certain. They breed in decaying wood and (according to Stuart Ball's key to Diptera familes) clusiid males display courtship behaviour on trunks and stumps, so that could be what you were seeing.
Entomologist and biological recorder
Lat/Lng: 51.9934, -0.652924
OS grid ref: SP925336