When I spotted these mini-beasties, they were doing a very good job of munching their way through about half the tasty young leaves of a Birch sapling. I knew they were sawfly larvae – that bottom-in-the-air pose is very characteristic of sawfly larvae – and, naturally enough, I assumed they were the Birch sawfly (Cimbex femoratus), but a quick image google showed I was wrong. It took a while to track them down but it seems these are the larvae of Nematus septentrionalis (formerly Craesus septentrionalis).
I was rather concerned for the health of the Birch as it is one of a relatively new plantation at the local country park but, when I revisited the site just a few days later, all but a couple of the larvae had disappeared. I don’t think they would have grown to pupation stage that quickly, so I assume a local bird or two had enjoyed them.