Tip the milk into the slow cooker. Cover and heat on High until the temperature of the milk reaches 82C, this will take a couple of hours. Turn off the slow cooker and allow the temperature to drop to 43C for a further 2-3 hours. Take a mugful of the warm milk and mix it with the yogurt then pour the mixture back into the slow cooker and stir really well. Cover, wrap the slow cooker in a big towel and then leave undisturbed for 9-12 hours until the mixture has set.
Eat on top of cereal or porridge, topped with fresh fruit, in marinades or drink in smoothies. If you want it thicker, for dips for example, line a large sieve with muslin and place it over a bowl, tip in the yogurt and allow some of the whey to strain off until you get the consistency of yogurt that you like. The longer you leave it, the thicker it will become. Store in the fridge for up to 2 weeks.