They are edible when they first start to grow, Hank, but it would be a waste to eat them at that time. I would use them after the five remaining lettuce plants and when you feel ready to start harvesting the new ones. Are they a variety that you can pull some of the leaves off but leave the plant to keep on growing?

Thanks B, I'm pretty sure they were Arctic King. I've never set any at this time of year before and I just wondered whether they'd grow. But they seem to be doing nicely and I put fleece over them Monday and Tuesday when it was very cold.
I just love my lettuce, salami,and beetroot sanswiches at any time of the year.