Caterpillar - but which one?

Caterpillar - but which one?

Observation - Caterpillar - but which one? - UK and Ireland. Description: This strange caterpillar was on a lichen-clad wall of a bridge over a burn. Its about 5cm in length, and seems to have good camouflage, with small green blotches (which have tiny black dots on them) along its body above each? leg. The body colour is the

This strange caterpillar was on a lichen-clad wall of a bridge over a burn. Its about 5cm in length, and seems to have good camouflage, with small green blotches (which have tiny black dots on them) along its body above each? leg. The body colour is the grey-brown of a typical twig on a tree (or maybe a plant with woody stem). Nearby trees were mostly willow,and conifers across the lane.