If your plants are suffering from frost damage when the cold weather comes in wintertime, then you need to purchase yourself a greenhouse heater. A decent heater for your greenhouse can be picked up relatively cheaply if you look about.
Of course, 'relatively cheaply' is different for us all and dependent upon your needs. Take a Bosmere 2000 Watt Electric greenhouse Heater - if you shop about you can pick one up for less than £30 which isn't too bad for a greenhouse of smaller proportions. For larger greenhouses, though, you could pay between £100 and £200 for a decent heater which pokes quite a hole in your pocket. It's significantly imperative, therefore, that you are sure of what you need for your greenhouse and your plants to guarantee you get the correct bit of kit.
The insulation on most plastic greenhouse have quite a low insulation value and so the purchase of the correct greenhouse heater is important to replace any lost heat. When further heat is needed to retain the temperature at the accurate levels, you're best off using a paraffin or electrical heater. A thermostat as part of the greenhouse heater you purchase is an important addition as it allows you to have exact control over the temperature in your greenhouse.
I'd rule out a gas heater as the initial cost in purchasing one can be a little too high. Don't take that as gospel though - gas heaters can, in the long term, turn out to be the most cost-effective option of them all.
The running costs of your greenhouse heater are an important consideration. Currently, the electricity cost in the UK of running a 1000 watt electrical heater would come out at close to 14p an hour. A paraffin heater, however, could cost about double that for the same period. That might have made your eyes swell - but the advantage of going with oil is that the heater will take time to cool and so you get "free" heat for a period of time after you've switched it off.
My own personal preference is an electrical greenhouse heater as I like the "cheaper now" option as I don't have a huge budget and with the inbuilt thermostats on most products these days, I can keep the temperature within my own greenhouse just right (and get your insulation right), but don't let that put you off a paraffin or gas heater if your need or budget stretches that far - each option does an brilliant job of keeping the temperature of your greenhouse just right and every dog has it's day..