Pour the milk into a large heavy-bottomed pan and heat over a medium heat, stirring continuously to prevent burning. Heat until the milk reaches 93°C (200°F) , just before boiling. Without a thermometer, heat until the milk is very hot but not yet boiling, small bubbles will appear at the edges.
Remove from the heat. Sit the pan in a sink of cold water and allow to cool to 46°C (115°F). Without a thermometer, the milk should feel comfortably warm rather than hot when you briefly dip a clean finger in, similar to a warm bath.
Place the starter yogurt in a large bowl and whisk in one cup of the warm milk until smooth.
Gradually add the remaining warm milk, stirring gently with a whisk until fully combined.
Cover the bowl tightly with cling film or a clean tea towel. Place somewhere consistently warm for 8-12 hours...an airing cupboard is ideal. Alternatively, briefly warm your oven to 50°C, switch it off, wrap the bowl in towels and place inside with the oven light on to maintain gentle warmth.
After 8-12 hours the yogurt should be set. It will be thicker than commercial yogurt and may have some clear liquid (whey) on top, either strain this off for a thicker Greek-style yogurt or stir it back in.
Refrigerate for at least 2 hours before serving. The yogurt will firm up further as it chills.
