As seen during our appearance on BBC Gardeners World we use garlic wash as a slug deterrent on the nursery. Please find below the recipe that we use and find very effective. The quantities aren't exact and you can always alter them as you wish.
Take 2 full bulbs of garlic and add them to approximately 2L of water in a saucepan.
Boil until soft and squash with the back of a fork to release as much juice as possible.
Sieve out skins and bits so you are left with a cloudy liquid concentrate.
Dilute approximately 2 tablespoons to 5L of water in a sprayer or watering can.
Spray or water over your plants once a week.
High dosage and more regular applications may be needed in wet weather.