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.

Store outside in a cool dry place or in the fridge. Can be used throughout the season, make a fresh batch every year.
Open a window because it's going to get garlicky!
We are currently on site at RHS Chelsea Flower Show. The nursery team are still dispatching orders as normal but please bear with us if there are any delays to enquiries during this busy time.
Many Thanks