[New shoot]

A lot of salad leaves are really cold tolerant, so the temperature slows them down, but they can cope.  Hearting lettuce would need to be a reasonable size before the cold really sets in, but the loose leaf stuff can be picked as and when it produces any leaves.  The slower it grows, the more bitter the taste, so keeping things in active growth with fleece can pay dividends.

It can get very stale and wet in the greenhouse over winter when everything is closed up and that can be a breeding ground for rot.  Space them well and be sparing with the water.  Helps if you can get any fleece up so it doesn't lie on the leaves.  I run some string from wall to wall and drape the fleece over like a tent 

[mumofstig]

Also opening the door for a few hours a day unless it is actually freezing or poring outside, stops the inside getting too damp