1. Reserve a few raspberries for decoration.
2. Place the remaining fruit in a blender or food processor and process until smooth. sieve to remove the pips.
3. Blend the cornflour and sugar to a smooth paste with a little of the grape juice.
4. Stir in the remaining grape juice and fruit puree.
5. Bring the fruit mixture to the boil, stirring all the time, and boil for 1 minute, stirring continuously.
6. Leave the mixture to cool, stirring occasionally. mix in the yogurt and transfer to four serving dishes or glasses.
7. Leave until cold, decorate with the reserved raspberries and serve.