Hi, I'm growing....
Sweetpeas, peas, cauliflower (all the year round is a good variety) cabbages, lettuce (winter density)
My greenhouse is 6x6 and unheated, by January I put some bubble wrap up, and place the plants on polystireen (cant spell that!) on the benches.
I have a parafin heater, but thats only used in April once the Toms and tender plants are in there, and only if a really cold night is forcast.
sure others will have more ideas... THREE greenhouses hey!!! :o