Yep, checked the vid card fan
It's still spinning merrily. Unfortunately it doesn't have any temp sensors on it, but FWIW I'll clean any gunk out of its heat sinks anyway.
I think I have an S3 PCI vid card lying around somewhere, so I'll give it a go if I'm feeling brave; I'll have to find something apart from games to stress the CPU though :)
Two out of three people wonder where the other one is.