Heating cable

Scarlet first you need a electric supply in the greenhouse, I made a wooden box 4.6ft. X 2ft. X 9inches high, the base of mine is made with 9mm. plywood so I added a sheet of polystyrene below that as insulation, line the box with polythene this is to keep the infill damp so you want it watertight, if you puncture it, seal it with waterproof tape, I use concrete sand as the infill medium and add a 2inch layer over the base then run a soil heating cable mine is 6mts. long (follow the instalation instructions on the box) then cover this with another 2 inch layer of sand, the temperature should be controlled by a thermostat and nowadays you can buy them with a socket into which you plug in the heating cable and it has a temperature probe fitted as well and you can push this into the sand or place in a pot of compost sitting in the propagator, once ready pourwatering cans of hot water over the bed, doing this gets the temperature up quicker, I strongly recommend sand as other flammable material can dry out and ignite, and always keep the sand damp which helps the heat distribution you should be able to buy the cable and thermostat in any GC or you can buy them on the Internet I have my propagator covered with a poly tunnel but will be removing that as it causes too much moisture in the atmosphere of the box and will replace with fleece
List of materials
Wood or plywood to make the box
Sheet of polythene
Washed concrete sand 4bags
Heating cable
Thermostatic controller
And most important, electricity