Thinly pare the orange and lemon zest, and set aside for decoration. Cut the fruits in half, and squeeze the juices into a jug. Set aside.
Heat the sugar and 500ml/16fl oz water in a heavy saucepan, stirring until the sugar has dissolved. Bring to the boil, then boil without stirring until a syrup forms.
Remove the syrup from the heat, add some of the orange and lemon zest, and shake the pan. Cover and leave to cool.
Strain the sugar syrup into a shallow freezer container, and add the fruit juice. Stir well to mix, then freeze, uncovered, for about 4 hours until slushy. Remove the half-frozen mixture from the freezer and mix with a fork, then return to the freezer and freeze again for at least 4 hours.
To serve, turn into a bowl and leave to soften for about 10 minutes, then break up again and pile into long-stemmed glasses. Decorate with strips of orange and lemon zest.