This fine specimen is a Puss Moth (Cerura vinula) caterpillar. It was feeding on willow next to ponds in Easington, Durham. They can also be found feeding on Aspen and Poplar.
It was a juicy 60mm x 17mm (approximately) The false eyes and orange ring on its head are there to deter predators. When disturbed it will raise its head and developed rear legs which it flails around (below). As an extra measure it can also spray formic acid. Nice.
The caterpillar turns orange then purple before pupating and overwinters inside a cocoon on tree trunks or fence posts.
Below: a picture of the puss moth when it emerges from the cocoon in the final stage of it’s life cycle. It’s covered in soft hairs (resembling cat fur) which give it its name.