Unknown Caterpillar

I am in the process of demolishing a lean to wooden shed as the roof is shot through and the wood rotting. That also gives the opportunity to move a couple of water butts. When I lifted the base of one I spotted this caterpillar hiding underneath:



I made a few attempts at identification but so far drawn a blank. Anyway, see previous post, it's not my month for getting identifications correct. Its length was somewhat over an inch, say about 35mm. After a short while it crept off to find another hiding place.