Well, turns out little Frostie had a nasty abcess on his shoulder so he went straight to the vet today. He was so good in the basket and car, not a peep out of him. The vet immediately fell in love with him as he behaved impeccably and let her drain his wound and take a blood sample (and temperature) with not one whinge.
He's got to wear a collar for a while and not go out so that might be a bit challenging, but I'm so glad we got him in time to sort him out. He must have felt better already because he went straight for the food bowl as soon we got home.