It is the caterpillar of the lime hawk moth, Mimas tiliae
As you'll see from that page, by virtue of its colour, we know it is ready to pupate, hence wandering around looking for a suitable place.
I suggest you put it back in the undergrowth, somewhere where it won't be disturbed between now and next May/June, when it will be ready to emerge as an adult.
We've taken your advice mike and taken the caterpillar to a local park here in Southampton so he can slip away and get ready for his big transformation into that beautiful lime hawk moth ... You've lite a spark in my step sons imagination and for that .... Thank you
You certainly need a lot of that to imagine what it must be like to go through the metamorphosis that butterflies and moths do. As a caterpillar, one day you find you've lost interest in food, and feel somewhat aimless. So you wander around, and get a vague feeling you ought to be looking for somewhere to kind-of go to sleep (the 'somewhere can vary greatly depending on what species you are). This'll do. Oi - what's this? Yes, I've shed my skin again, but now I've go no feet. ...And no mouth. ...And I can't see! Well, best make the best of it... Inside your pupal 'shell' you then break down into a gloop (this must be a sort of dream-time). The days and weeks pass, and gradually your gloop reassembles itself. Eventually, you start to wake up. But you're trapped in shrinkwrap! Like trying to wake yourself from a nightmare, you wriggle and struggle... Your pupal case splits and you can crawl out - phew! Now what? I've got wings and antennae and legs. But I mustn't get in a flap; not yet! Panting from the effort, your blood circulates through the veins in your crumpled little wings, expanding them until they are big and flat and... ...in desperate need of being flapped. Hey - geez - I'm airborne - cool! And I'm awfully hungry. I bet there's some nectar in that flower...