Had you thoughtof a fan heater - they have froststats and help to keep a good circulatin of air.there are models designed for greenhouses.

We use a greenhouse model fan heater in our front porch in winter as Bulba brings in a lot of frost tender plants and this works well for us.

Have you considered a paraffin heater? We have a greenhouse about the same size as yours and it works really well. Obviously don't leave it on when you leave!

Joan

The paraffin fumes aren't good for the plants and it creates condensation. Also I would have thought the whole idea was to be able to leave on when you weren't in the greenhouse.

Be aware of electrical safety when getting a cable from source to the greenhouse! Make sure that the cable can't get damaged and that the plugs and sockets don't get damp.

My dad had a cylindrical plug in heater which used as much power as a light bulb (which is still useable) and which cost peanuts to run. You can still buy them.

Have you got somebody to take a cable from the house to the greenhouse? I don't think you have power in there now?

Bulbaholic has given advice that can save a life. I would add that any outdoor electric device be connected to an outlet that has what we call in the USA "ground fault protection" which means the outlet immediately shuts off if a electrical short is detected ...whatever the reason. These outlets and plugs are low cost and many need no special installation.

In Myouse's case the cable would be best buried to avoid tripping problems and lawnmower accidental damage. It should be installed by a professional who should use a specially designed waterproof socket with the fuse up in the house.