Well actually it turns out to be something quite common. I went out yesterday morning with my secateurs to trim little extraneous bits off my crab apple tree and nearly cut a caterpillar in half! Luckily it dropped off as soon as I touched it with the tip of the secateur. But the camouflage was amazing.
I've looked it up and its a Biston betularia (so should never have been on my crab apple at all except that it fitted in so neatly). And the butterfly it turns into camouflages just as well. There are apparently several forms of this butterfly. I've downloaded this second picture from the web, I hope no one will mind, the first picture is mine.