Reserve about 50g of the mixed fruit for decoration. Put the remaining mixed fruit in a saucepan with the water. Bring just to the boil, then reduce the heat and cook gently for 5 minutes or until soft and very juicy. Stir in the sugar.
Remove from the heat and leave to cool slightly. Pour into a food processor or blender and purée. Press the purée through a sieve to remove all the pips. Alternatively, just press the fruit through a sieve to purée it. Set aside to cool completely.
Combine the yogurt and orange zest and lightly whip together, then mix in the cooled fruit purée.
Spoon into dessert dishes or goblets. Chill well before serving, decorated with the reserved berries and orange zest and mint, if using.