I was annoyed to see the damage and then I noticed the small seedling right next to the munched lettuce - its a very tiny lambs lettuce. Looking around I found several more of varying sizes and there doesn't seem to be any sign of slug damage.
Last summers lambs lettuce set seed which germinated in the autumn but didn't really start growing until the spring. I was picking the leaves in mid March. This year the germination is much earlier and it looks as if I will be picking leaves this summer. I don't know whether slugs will eat lambs lettuce but at the moment they seem to be leaving it alone.
So it looks as if the main constituent of my salads will be lambs lettuce rather than my favourite red oakleaf. It makes more sense to eat what I can grow easily rather than struggle to grow slug food!