Silver / Grey caterpiller on a young hazel tree.
No identification made yet.
No interactions present.
This looks a bit like the White Alder Sawfly, Eriocampa ovata, but as its name suggests that one feeds on Alder not Hazel. Do you have any photos of the leaves of the tree?
There's some more information about White Alder Sawfly at:
Entomologist and biological recorder
Lat/Lng: 51.3218, -0.6185
OS grid ref: SU963589