Stir the sugar into the measurement water until it has dissolved, then leave to cool.
Mix together the sugar syrup and champagne. Pour it into a shallow, nonstick baking tin so that it is no more than 2.5 cm (1 inch) deep.
Freeze the mixture for 2 hours until it is mushy, then break up the ice crystals with a fork. Return the mixture to the freezer for 2 more hours, beating every 30 minutes until it has formed fine, icy flakes.
Spoon the granita into elegant glasses and top with the raspberries.