The poor old thing looks as though it's seen better days Steve, doesn't it?
Have a look at the Pine Hawk moth. This is a large European moth, wingspan 7.8 - 8cms, forewing grey-brown with blurred bands, delicately mottled, hind wing brown, fringes narrow with alternating dark brown and white - also blurry.
I can't find anything else in my books as large with that colouring, so perhaps someone else will come up with something if you don't think this is the right one.
Thanks very much, Cetti. Yes, that does seem to be the one. It really is in a sorry state (no scales on the wings and no fur on the back). It is still alive more than 24 hours since I first saw it.
I put it on a sheet of paper in the bright sunshine to take the photo and it really didn't like that. It's now in a safer place but I don't think it's much longer for this world.