Found sitting on a pigeon feather, on a path underneath an Oak tree. Bright green body approx 25-30mm in length. Many black 'spikes' along the body, forked at the ends.
No interactions present.
Oak Sawfly. Thanks very much David, has been bugging me for days! (Excuse the pun.)
Lat/Lng: 54.2, -1.1
OS grid ref: SE6183