Place the water and sugar in a medium saucepan over low heat and cook, stirring, until sugar has dissolved. Set aside to cool slightly.

Combine sugar mixture with juice in a metal container. Cover with plastic wrap and freeze for 4 hours or until set. (I found I had to freeze it over night in our fridge freezer. A deep freeze may take less time).

To serve, rake the granita with a fork and spoon into serving glasses.