What natural slug deterrents actually work?

We tested a whole load of commonly cited slug and snail deterrents to see what happened. The answer? Not much!! When slugs and snails are travelling along a horizontal surface then nothing much will stop them, as they an just put out a thick layer of mucus that stops them contacting anything below. Slug rings work as a vertical barrier as the slug or snail detests copper, but if it puts out a mucus layer to protect themselves.... they slide off!