The 'best' heater would be an electric fan operated one with a thermostat set at 7C with a view of the whole greenhouse staying above 5C. There is no need ot set it above that as the plants do not need it with the amount and duration of light in winter. The fan helps spread the heat around the grenehouse and not at one spot. They are expensive to run but cheaper than buying new plants! Bubble wrap will also help keep the heat in.
I do not have electricity down in my greenhouse and use 2 paraffin heaters. It is a matter of switiching them on and hoping for the best tbh as there is no control.
I find a wooden floor 'warmer' than stone, but if the sun can get to a floor stone then it is possible that the heat is stored in them and released when dark.
If you want to be adventurous then you can try a heat sink system - you will need to google that as it is too complex to explain.