Lackey Moth (Malacosoma neustria)
With a mask-like blue face that wouldn't look entirely out of place in a horror movie, the caterpillar of the Lackey moth is definitely one that's unlikely to be forgotten once you've seen one.
This fully grown individual was in the process of finding a suitable place in which to pupate. Part of a lifecycle described in more detail here:

The lackey moth is common across southern Britain and central Europe. "Malacosoma" species are notable for their caterpillars which are brightly coloured and form silken tents to regulate their temperature. "Malacosoma neustria" caterpillars are brown with blue, orange and white stripes. The adults are a fairly uniform brown. The larvae feed mainly on trees and shrubs from within their tents.