Entomologist Richard Jones replies:
It was probably the fullgrown larva of the large yellow underwing, Noctua pronuba, though there are plenty of very similar-looking muddy-brown-green-mottled caterpillars. Fully grown by December, they usually burrow into the soil to overwinter, but do not pupate until spring.ng. Finding large larvae crossingg tarmac is common as they search for some soft soil in which to self-inter. Mild weather throughout the UK has meant that some caterpillars have gone on feeding into January, and only then thinking about seeking subterranean shelter.