No interactions present.
This is a sawfly (Hymenoptera rather than Diptera) - possibly a Dolerus species
National Recorder for Terrestrial Heteroptera (shieldbugs & allies)
It's a sawfly, and I think I can see just three segments to the antennae, two small ones at the base and a much longer third segment making up the majority of the antenna. This would put it in family Argidae, and it looks suspiciously like Arge berberidis, the Berberis sawfly (but I don't know whether there are other species that look similar).
Was there any Berberis nearby? If so, had there been any feeding damage to the leaves earlier in the year?
Arge berberidis is another recently introduced and invading species, for more info see:
Entomologist and biological recorder
no not sure of any berberis nearby was seen on lemon balm though just seemed to be resting
Lat/Lng: 51.4, -2.6
OS grid ref: ST5970