Have just read an article in the Daily Telegraph on cottage gardens and thought I would post this excerpt which refers to delphiniums:
"To counteract slugs, he buys aluminium sulphate from a chemist and puts two tablespoons in two gallons of water and applies it around the plant when the leaves emerge. The advantage over slug pellets is that this penetrates into the soil. It acidifies the soil and is used as a hydrangea colourant to keep the flowers blue.
Nick Langdon of the renowned nursery Blackmoor and Langdon (blackmore-langdon.com) recommends applying it in the autumn, too, as slugs lay eggs in the crowns of the plant which will devastate young leaves in spring."