While I was watching the Campodorus (the primary parasite) continuing to parasitise the sawfly larvae, I noticed this bright green caterpillar on an upturned leaf on the same willow. The spiky 'tail' suggested a hawkmoth of some kind, but I wasn't sure which one. A fairly quick search on the excellent ukleps.org turned up a version of the Poplar Hawkmoth. This was a bit of a surprise, but a little research shows that it's often found on willow, too.
New to me, although it's common enough.