Thanks, Jamie - the original new one I got for this set up is the same as this:

http://cgi.ebay.co.uk/HABISTAT-300W-...QQcmdZViewItem

and that seemed to fluctuate all over the place too - I take it that isn't a proportional stat either - could you recommend me one?

EDIT: actually think I've just found one - it's this
http://cgi.ebay.co.uk/HABISTAT-PULSE...QQcmdZViewItem

~Sue