I'm not really any good at fly ID.
Morellia (one of 3 species).
ah, I see Mydea got there before me.
These striking Muscids are common on anything flowering, especially Umbellifers, I'm not aware of a particular attraction to Fleabane.
Edit: compare the tachinid-like median vein with the specimen here, as well as the distinct dusting on the thorax
latest pics and diptera videos
I assume there are no features that can be seen in this picture that would allow a species level ID.
Lat/Lng: 51.540316447216, -0.32921433448792
OS grid ref: TQ159837