Spotted this chap just now crawling across my patio. No idea what he is and net searches have proved fruitless, need to know what he is and what he eats, so I can put him on the right plant!
I'm not sure what this will become, it might help to know where you live, as there are sometimes folks outside the UK posting for ID's.
Its also worth noting that this behavior is generally down to them looking for a place to pupate, my guess is that if it was walking across a patio, it's likely looking for dirt or leaf-litter, try putting it into a good sized tube with both and see what it does. It is definitely a moth, and many moths will do this when they are ready to pupate.
After all it didn't get to this size by not eating what it wanted to eat, looks quite plump in your photo.
Hope this helps,
Aha, so it'll be a moth. I'm in north east wales, never seen one of these fellas before, but I'll see what he turns into. There's space in the garage alongside the elephant hawk moth!
It's a caterpillar of the Dark Spectacle Abrostola triplasia. It feeds on Nettle and Hop.