Further to my reply to Guy's post I meant to mention that I have also located the following (sorry can't do tiny-urls)
http://www.ecs-marine-equipment.co.uk/marine-equipment/labcraft-orbit-series-orbit-item-12946.html That's the kind of unit I'd like, with on-unit switching but it's probably not quite big enough, diametrically speaking. If anyone knows of something like this but bigger pls let me know. Beeky
