Large numbers of this tiny, yellow fly on Hawthorn, today.
No interactions present.
Great photographs - these look very similar to a recent observation of small flies on sycamore tentatively id'd as Agromyzidae (Leaf Mining fly) by David Notton http://www.ispot.org.uk/node/264879?nav=users_observations
They do look similar. These are Chloropidae, with the shining ocellar triangle. That is the right family for Thaumatomyia.
My Flickr photos...
Lat/Lng: 53.8833, -0.682
OS grid ref: SE867438