Andy, recently did a lot of research on heating my workshop and have come to the conclusion that space heaters aren't a good idea, if your talking about the tubular kerosene or gas fired ones they will make the place quite damp, you would probably be better off with some second hand industrial IR heaters, cheaper to run and only produce heat where you need it not the rest of the airspace.
Something like this
http://www.ebay.co.uk/itm/Infrared-H...item1c2f31c408
HTH