4
Well, guess what? It was a browser issue!
I ended up completely removing the forum and installing it again on my site. Trying to add a moderator still didn't work!
I then tried it in Internet Explorer -- presto! Now it works.
So, in summary:
IE 6: Good
Firefox 1.5: Bad
Well, good and bad in this single example anyway ;)
Thanks everyone for taking the time to look though...