1. Crush the raspberries with a stainless steel or silver fork in a china or glass bowl.
2. Mix them with the sugar and put them on one side for 10 minutes. The sugar on the raspberries will draw out juice and bright colour.
3. Beat the double cream until it is thick, then, tablespoon by tablespoon, beat the yoghurt into it. The yoghurt will not thin it down; if you beat it each time you put in a dollop of yoghurt, it will bulk back up again. It allows you to have twice the amount of cream, as it were, with only half the amount of fat.
4. Swirl the raspberry and sugar mixture into the cream and yoghurt - do not mix it so thoroughly that it looks like pink yoghurt, swirl it in so that it is marbled.
5. It looks beautiful. Pour it into glasses so you can see the marbled effect, decorate each one with a sprig of mint and put to set in the fridge for 1-2 hours.
6. You can leave it for up to 12 hours without any deterioration.