How easy is this!
1 pt milk (I use semi-skimmed)
1 tbsp milk powder
1 tbsp yoghurt (Natural yoghurt either saved from the last batch or bought new. Must be 'live')
Put milk into a pan, stir in milk powder, bring to boil. Once it has come to the boil let the milk cool down to body temperature (it should feel slightly warm) and stir in the yoghurt. Leave in a warm place overnight and it should be set in the morning.
I have been using the piece of culinary archaeology above, you can put warm water in the base to help things along, but you can just use a jug or get a fancy yoghurt maker. Since it's been really cold I have been thinking of using a plant propagator, as even our airing cupboard doesn't get warm enough.