I have had mixed results with winter lettuce in the past due to weather conditions. My first attempt at protection with fleece failed miserably, it just encouraged slugs and fungal activity. If I leave them outdoors and the weather turns very cold; growth stops and the plants suffer. My current method is; sow in pots in September with minimal protection mainly to keep heavy rain off, the pots are stood off the ground (slugs!), plant in soil under glass when greenhouse becomes available (November), water sparingly. This winter I am trying some in 9cm sq pots also under glass. I dont have a major problem with slugs and snails for now and I still have some “Marvel of 4 Seasons” in a well ventilated cold frame but it seems to me that wet conditions cause most problems. I have been growing “Winter Crop” for a couple of years now, a nice lettuce and its done very well.
Cheers
Pete