If you are starting to shiver then so are your plants! Greenhouse heating is important. Paraffin heaters are the long standing gardeners favorite.
You need to protect your tender plants and plant plugs which you have lovingly cared for and nurtured throughout the winter months.
Greenhouse heating can be by various methods- If you have electricity to your greenhouse you can consider a fan heater, but this isn't the cheapest way to protect your investment. Ensure it doesn't come into contact with water either.
You can insulate with bubble wrap and/or fleece (remove fleece during the day - it blocks light and ventilation). You may want to use bubble wrap around your expensive external glazed patio pots as well to help prevent frost damage.
Paraffin greenhouse heaters are tried and trusted by gardeners for many years. We stock a range, and some only need refilling every 3 or even every 7 days.
Your plants don't need high temperatures in the winter. Keep your greenhouse at a minimum of 2c. Most plants are happy at 7c, but very tender ones may need a higher temperature. As always with gardening it's trial and error.
Position your paraffin heater where the air can circulate, and check your thermostat. Make sure the heater is on a firm, flat surface, and nothing can fall onto it.
Ventilation when weather permits is still a good idea.