Approx 10mm long, one of a number found, many were on trunks of nearby oak trees. Some much smaller
No interactions present.
Yes, definitely a hoverfly, perhaps in genus Melangyna or Meligramma, both of which look a bit like this and live in trees, but that's only a suggestion not a confirmed ID!
Entomologist and biological recorder
Lat/Lng: 51.9, -2.1
OS grid ref: SO9324