I recommend a little devil propane heater, we had one of these in the workshop for the last two years, heats 1000 sq ft in about 1/2 hour if you use a large propane cylinder to get decent gas pressure. I think we paid about £50 in the preowned section in Machine Mart.
http://www.machinemart.co.uk/shop/pr...rkshop-heaters
This year we got fed up with the cold and the fume smell from the gas so we got a fixed powrmatic NV, but we are running a business from the premises!