Put a heater inside your greenhouse
Install a fan or bar heater in the greenhouse to prevent frost damage in a cold snap. Heat mats on the bench are ideal for young plants to stand on, too.
Make sure your electrical supply meets the required safety standards for outdoor use, otherwise, choose propane gas heaters or paraffin burners and follow the instructions carefully. Whichever you use, check them regularly, don’t have them in direct contact with plants or structures, and use horticultural fleece as an extra layer over the most vulnerable plants.