I too have a greenhouse. I do not have a heater. What I do however is that, during the day, if its going to be warm, I open the vents, but when I go in the evening to water the plants, and I know its going to be a bit chilly, I then shut the vents, rememberting to open them the next day. Toms and cukes (my US friend calls cucumbers "cukes") are fairly robust. However, I must admit I've not had a successful year with cukes - last year, my first time of growing them, they call came along and I got lots of them. Thats the way sometimes with plants!!