Lou,

Unless you have a very serious slug problem I think it best not to use slug
pellets. There are quite a few predators looking for slugs and it would be
a shame to kill them with poisoned slugs: but of course,(!!) the slugs are
under fleece so the predators might go to your neighbour's garden! Is that
good news to you?

Keep the lettuces well watered and they'll grow faster than slugs can eat
them and do not forget to plant a few more seeds every few days.

Regards

Ron