Apparently snails and slugs don't drown. I found another snail, and put it in the trug and watched it this time. It definitely crawled back out - slowly mind, no rush. So I put it back. It crawled out again. And the third time. Maybe even the fourth.

Time to come up with a new slug and snail plan. Stamping on them is NOT an option.  

Another mystery however - what is pulling my modules of peas and mange tout out of the ground? And why?