Back home, let the ready-made yoghurt come to room temperature, pour the milk into a saucepan and whisk in 25g powdered milk (if using) for every 500ml of milk.
If this is pasteurised, heat it to 85C, stirring occasionally, then leave it to cool to about 46C (this process results in thicker yoghurt, as well as killing some unwanted bacteria).
If UHT, simply warm it to 46C. Then whisk or stir in the live yoghurt – about 3 tbsp for every 500ml.
Before the mixture can cool, pour it into your warmed Thermos(es) and screw down the lid(s).
Leave it for eight hours or so – or longer if you like your yoghurt thicker and stronger-tasting.
If it either tastes or smells off, chuck it away and start again.
Otherwise pour it into clean jars and store it in the fridge.
If you're happy with how it turns out, you can use a little of it as starter for your next batch.
Freeze this if you won't be using it within the next few days.
v