Once the slugs have eaten all your lettuce seedlings, is that the end of them completely?

[Caligula]

Will they ever grow again, or should I just give up on that bit and plant the tomatoes out there?

Yes, it is the end I am afraid BUT you can resow and cover seedlings with cut off plastic bottles which will save them from the slugs.

Good luck and don't let the slugs put you off.

Cheers. Will try the plastic bottles thing!