It was large, very bright green-yellow, with a black stripe on its back. I found a plastic bottle around - a good use for litter - and put it in it until I got back home to take a photo. It was next to a fallen lime leave and I got some fresh lime leaves as a background for the photo. The result is quite nice for the strong contrast of colors.
I wonder if it was looking for somewhere to pupate. I have seen Lime Hawkmoth caterpillars at this time of the year as limes are a common tree in my street, but it is the first time I see this species. I have put a thread in the Wild About Britain site to see if somebody helps with its ID.
Shockingly, and brilliantly, the 'caterpillar' turned out to be a larvae of the large alder sawfly, Cimbex connatus, thanks to the WAB site for identification. WAB is a fantastic, very rewarding place to identify bugs. Usually it is a matter of hours until a discussion arises about an unidentified bug photo is posted, often involving experts in the field.Sawfly larvae are remarkably similar to butterfly and moth larvae - to me a great example of convergent evolution-, but they can be told apart for having a larger number of 'false legs' and other details. They are hymenopterans that lay their eggs into the tissues of plants. It turns out that this sawfly species is quite rare in the UK, with only a few records, and that it had probably fallen out of alders, next to the lime tree. Alders do not lose their leaves until late in autumn, or even early winter, so this bug had still some eating to do.
A great find for such a wet morning.