@Wozzaaah
Do you have a compost bin/heap?
If so, make it a hot composter, Googlr will show you many ways.
Place a old car radiator inthat heap.
Pipe it to a second one in the green house. The heat will travel through the water, by convection and keep it above 0deg C
In the day it works backwards and heats the compost, allowing it to break down faster.
