Caterzilla

I had a puncture yesterday for the first time in four years. I discovered, as a result, that my Guardian car doesn't have a spare tyre - just a bottle of latex and an electric pump which was of no avail against the tear in the tyre wall. My frustration was relieved by discovering this caterpillar on the roof of the car while I waited for the AA (who came very quickly, I might add, and ATS in Bradford did a great, speedy tyre replacement too). The car in the picture isn't mine. Morris Travellers sensibly did have spare tyres. I added it for scale. It's Matchbox rather than Dinky, I should add.
Does anyone know what the caterpillar is? I am checking in my usual rather purblind ways through my various books and online, but there seem to be an awful lot of green caterpillars like this. I wondered about the Speckled Wood, which has featured much in this blog, but I don't think it's quite right. I kept it overnight but it clearly wasn't happy in captivity, in spite of lush helpings of privet, grasses and willow. It crept as close as it could to the lid of the plastic raspberry box and was gripping on firmly by its fork-shaped tail (see detail). So this morning I liberated it into the wider world again.