Isn't he extraordinary? He stayed just like this, reared up like some prehistoric mini-dinosaur, no doubt in an attempt to make us fear for our lives. He needn't have worried - we certainly weren't about to eat him.
Fortunately, given his size (about the thickness of my ring finger and almost as long) he's not a garden pest. I discovered from the outstandingly good identification site UK Moths that he's an Elephant Hawk Moth caterpillar, and eats rosebay willowherb - which, owing to the fact that I'm seriously behind on the weeding at the moment, I happen to have a patch of at the end of one of my borders. So off he went to feast on my weeds till he turns into a lovely big moth with pink-striped wings. Who knows - if he finds another big pink-winged moth I might I no longer have a willowherb problem. Now that would be a result.