A source of heat will extend your growing season and increase the range of plants you can grow. If you have a power supply, a heater with an automatic thermostat will do the work for you. To keep the greenhouse frost free in winter, aim for 4°C or above. If you don’t have power, you can use a greenhouse paraffin heater, although these need attending to usually twice a day. You can also use a propagator as a 'greenhouse within a greenhouse', providing added protection.
Maximum-minimum thermometers are invaluable – but buy two so you can monitor temperatures at either end of the greenhouse, and then position your heater where it will be most effective.