Hi Rhys,
So sorry to hear about your lettuces. I know for a fact when it rains a fair bit, the slugs seems to appear at our lettuce beds located in the half shaded area as opposed to the lettuce beds located in the drier in full sun location.
We haven’t had a whole lot of rain, hence are lucky that the lettuces haven’t had much of a damage. While I was picking the lettuces in the half shaded area, there were some baby slugs present especially.