Photograph: Martin Poole
Combine all the ingredients, pour into a 1 litre plastic container, cover and freeze for 5-6 hours. Whisk or beat well with a fork every 2 hours.
Allow to soften for 20 minutes in the fridge before serving.
Get ahead: make a few days in advance.
Kitchen secret: for an altenative, try blending the yogurt and icing sugar with some frozen berries and 1 teaspoon of vanilla extract, then sieve before freezing as above.