Have a look in your greenhouse after dark, that's when you'll catch the little t--ts at it.
I should have pointed out that over the fence is a large field...I would never throw snails into another person's garden, that would be too cruel
I do often do the bucket of salt water thing at night with a torch. Also, it is worth finding the nooks and crannies where they hide during the winter and visiting that spot around late Feb.
Put a few slug pellets under the bottom shelf of the plastic greenhouse, you will never catch them all by hand. Being able to afford new seeds is one thing, but you will run out of time shortly.
Lyn, that is a very good wheeze