Have a lean-to greenhouse! They have the advantage of getting some heat from your house in winter, either heat that would be lost, or if it is very cold, you could open windows into the greenhouse to let your house heating system heat it. Much simpler and less dangerous than any greenhouse heater. On warm sunny winter days, the greenhouse will actually add heat to your house. And Humidity. And Oxygen.
These floor standing Palma Electric stainless steel heaters are robust, compact and powerful with an output of 2kW and an air circulation rate of 400m3 per hour - stats usually associated with much larger heaters. http://www.harrodhorticultural.com/palma-electric-greenhouse-heaters-pid9175.html Palma Electric Greenhouse Heater - Harrod Horticultural
Keep your greenhouse frost free this winter! These Frosty Greenhouse Heaters from Bio Green are extremely efficient, thermostatically controlled gas heaters for creating the ideal temperature in your greenhouse to grow year round and stop frost damage to plants in winter http://www.harrodhorticultural.com/bio-green-frosty-greenhouse-heaters-pid9170.htmlBio Green Frosty Greenhouse Heaters