Mr Happy - have you actually seen the slugs eating your cat food? If not, one possibility could be that you have enticed your hedgehogs for a main meal, and they are using the slugs as starters or pudding (especially if they are marinaded in Guinness) Have you thought about putting down garlic butter for the gourmet predators.
Actually one of the warnings given about slug pellets is that they can cause problms for hedgehogs who eat the treated slugs.
Geese or ducks also eat slugs, although for myself, I'd rather stick to the beer.
Quack!
Geoff.