Small and slender, around 10 to 15mm long
No interactions present.
It looks like a young geometridae larva that gets considerably bigger. A look through early instar pics of geometridae larvae in ukleps (online)might produce an ID.
Thanks to both of you for your input.
Just had a quick look at UK Leps, brilliant site ! Looked part way through the geometridae family, will look at the rest shortly/
Lat/Lng: 52.9623, -1.4779
OS grid ref: SK351406
Large country park much of which is a designated local nature reserve. The caterpillar was observed on a fence at a woodland edge.