Measure the water and sugar into a saucepan. Stir over low heat until the sugar has dissolved. Add the finely pared lemon rind – use a vegetable parer and pare off only the fine outer yellow zest. Bring to the boil and cook rapidly for about 5 minutes. Draw off the heat and allow to cool.
Add the lemon juice and strain the mixture into a basin. Add the unbeaten egg whites and whisk with a fork to break them up. Pour into a plastic freezer container and freeze until the mixture is mushy – about 2 hours.
Spoon the partially frozen mixture into a chilled bowl and whisk until smooth and white. The whiteness comes from the air whisked into the mixture. Spoon the mixture back into the container, cover and refreeze.