We took the cat to the vet this morning and saw him off. Firstborn is home sick (probably Strep - will we ever be rid of this wretched disease?) so it was everybody except Secondborn. It's probably for the better that he was at school.
It was definitely time. The cat was essentially immobile, so much so that the vet didn't bother giving him a sedative to prepare him.
Definitely time, but it sucks.