Sterilise the yoghurt jars by washing them in hot and soapy water, rinsing them, and then leaving them to dry in a low oven at 150°C/300°F (without fan). Cool the jars before using.
Whisk together the sweetened condensed milk and the boiling water in a large jug.
In a medium bowl, whisk together the yoghurt and milk. Then pour in the sweetened condensed milk mixture and stir everything together.
Fill the yoghurt jars with the mixture. The easiest way is to pour the mixture back into the large jug or to use a bowl which has a spout.
Place the yoghurt jars in the yoghurt machine and use as per the manufacturer's instructions. With my Tefal La Yaoutière, it requires 45 minutes with the machine turned on, after which time the machine automatically switches off and is left unattended for 8 hours while the yoghurt ferments. After this time, the yoghurt should be quite firm, thick and creamy in texture. If you prefer a thicker texture, leave it to ferment for another 1 or 2 hours.
Remove the cover of the yoghurt machine and cover each yoghurt jar with a lid or some plastic wrap, and place the yoghurt jars in the fridge for a few hours or overnight to set.
The yoghurt keeps well in the fridge for about 1 week.