In a saucepan heat up the milk slowly but steadily, until just before it simmers. You don’t want the milk to boil but it should be very hot and steaming. Look for steam to rise up from the milk and little bubbles that accumulate along the sides of the saucepan. That’s a good sign to take it off the heat.
Leave the milk to cool down until it’s lukewarm. If you stick a clean finger in the milk it should feel only slightly warm to the touch.
Add in the spoonfuls of yoghurt and mix very well. Pour the mixture into a sealable jar and seal it.
Keep the jar in a very warm space.
You can make a little hot spot using a supermarket chiller bag. Inside the bag, add a large sealed jar with just boiled water. Wrap the yoghurt jar in a tea towel and then add to the bag and seal the bag up tight.
Leave to sit for 5-8 hours until thick. If you want greek-style yoghurt you can strain it now in a cheesecloth to seperate the yoghurt curds from the whey.
Store in the fridge for up to a week and save 5 spoonfuls of yoghurt for your next batch.