Thermo cube.

I just purchased one of these for use with the heater in the greenhouse. It will turn on the electricity at 2C and off at 7C, which is ideal for my purposes. To test it before I actually use it I plugged a small night light into the unit so I can visually check from the house and also check with my remote thermometer. The cost was only $12 US. Maybe available in the UK also.

 

Using the IP camera installed in the greenhouse and the remote thermometer in the house I was able to monitor the device; sure enough the light was turned on at 2 degrees C. It is nice when something works like they claim it would, and, I expect to reclaim the cost of the device in electricity savings.