Photo'd a couple of hoverflies on the same tree saps run in Georgia last week and neither of them are on the Georgian Syrphidae Checklist run by Ilia university, Tiblisi. So they are probably zooming off to the very tree as I type this, see below. Thanks to Gerard Pennards for identification help.
update -not thought to be any of the british species - new for Georgia
New for Georgia
Also had Eupeodes Corollae [below] and Episyrphus balteatus.