The only Solar option that might work with a timer is also electrical in nature, and would be subject to the same restraints, though lower voltage might make it somewhat safer. The alternative is a solar hot water system, in which black metal trays with pipes running through them are mounted on a separate roof. Water circulates through these collectors, and through a large, insulated water tank filled with stones, to store heat. At night, the warm water circulates through tubing in the floor, warming the greenhouse. It still requires electricity to run the circulating pumps, but no electricity enters the greenhouse. A simpler system would be to use a high efficiency water heater to run water through those self-same tubes. That may actually be more effective, considering how gray the UK can be in winter.

Hi, I would suggest a propane heater, which will have a thermostat to control the temp, and if you get 2 of the very large gas cylinders, and an automatic change-over valve, then you would be able to go away for a month or maybe more depending on how cold it got, all you would need is either someone to water for you, or some sort of a drip watering system, and a large enough water tank to last the time you'll be away, Derek.

Note that, unless vented to the outside, propane heaters produce minute amounts of acetylene gas, which can affect the growth, flowering, and maturation of many crops.

Hi Tug, I didn't mention that, because that information comes with the heater when you buy it, but you're quite right of course, Derek.

Depends on if you are shopping for it as a greenhouse heater, or just as a heater (unspecified)