I got a diesel cab/camper van heater off ebay, about £70. I've divided my double garage into a smaller 5 x 3m workshop and it warms that up from 2 to 20 degrees in about 10 mins. If I reduct to th E main garage it struggles to get to 15 degrees but that makes working out there quite comfortable. With the oil pump on full chat (so max output) to get the place warm then throttled down, I reckon it uses about a gallon of diesel a day. I get red diesel off a farmer mate of mine so it's about 80p/litre. It sucks combustion air in and exhausts to the outside and recirculates warm air.