It is the caterpillar of an angle shades moth, Phlogophora meticulosa.
They are usually green, but various brown shades occur.
Here's a reddish brown one -
Thats great thank you.
What should I do with him/her? (we're calling him Andrew for some reason) he seems 'still' and we thought he was dead at first and rescued him from a puddle but he is alive... just very still. We've put him in a small cardboard box & are leaving him alone for now. Is it best to find somewhere dry in the garden & just leave him alone?
I suggest you put Andrew back in the garden; anywhere under cover of some herbage (they have a wide variety of plants in their diet) but ideally where you found him. Either he will get back to his routine of feeding and resting (often under cover of darkness), or he will prepare to pupate, or he will become food for something else. If you keep him in a box, he may sulk and die. At least if he dies outside, he will probably do so in providing food for something else interesting/nice.
Then you can keep an eye out for an adult angle shades moth; they have lovely cryptic patterning.
And if you see one, maybe you'll wonder if it is Andrew.