Ok,
ronv: um, no idea but it does work happily with a dimmer.
RCinFLA: I did have a problem but not quite what I expected. The fan starts and runs happily at very slow speeds and will sit there for hours, but if I turn up the speed the dimmer will cut out after a while (10 mins or so)! Turn it off and back on again and it will go for a short while again. I'm guessing there is less load when it is at higher speed and this is enough to trip out the "intelligent" part of the dimmer.
MikeMI: The solution to the problem!! now my workshop and spraybooth are a lot better ventilatled and I can breath sort of normally again.
Thanks Guys.
Hobbit.