Lettuce in the greenhouse

yes, but it depends which varieties you have sown, whether they will put on much growth between now and spring, or just bide their time and wait for warmer weather

It's important to open windows at every possible opportunity or they get mildew

My summer sown lollo rossa are doing fine and supplying plenty of leaves for sandwiches. The AYR i sowed a month or so ago are looking pathetic.

Keep the air circulating as mum says. I had a bit of bother with mildew but I lifted the affected plants and things are OK now. My greenhouse is open all day but closed at night and will be until we get really cold weather.