The 'shredding' you describe sounds to me as though little slugs are the problem. Check after dark, especially this warm weather. Two tips;- 1. Water in the morning, as slugs and snails usually feed at night, and if the ground and plants are wet it makes it doubly easy for them to get about. 2. I don't use slug bait of any kind, but if you do, DON'T put it around the plants, put it in the dark corners and hiding places where the snails and slugs spend their days holed up, so when they come out to feed, breakfast/supper is right beside them. Why draw them to the plants with bait, where they can choose their meal? I'd choose fresh leaf over dried slug pellet!
Hope this helps

Worthy1's DEFINITELY on the right track here, only my iris munching fiends are snails. I go round on a very regular basis, pull the little (well, actually, round here, HUGE) b****rs off and feed them to the ducks. Doesn't stop them entirely, but it's a case of damage limitation.