...big thanks to Helen Foster and her colleague Andy (sorry Andy, no surname). 
The problem was the one of my categories had an & character in it - replacing these with the & character entity fixed things. I think I'd be staring at it for too long, since I couldn't see this even though it was right in front of my face! The definition fields were also full of <p><strong>Name: Ian Usher</strong><p> sort of stuff so I was even less likely to notice the stray '&' characters floating around.
At least, that's my story and I'm sticking to it... thanks Helen!