In a saucepan, stir together the diced lemon zest, 250ml water and sugar. Bring to the boil, then reduce heat to medium and simmer for 5 minutes. Remove from the heat and allow to cool.
In a large jug, stir together the lemon syrup with zest, lemon juice and sparkling water. Pour into an ice cream maker and freeze according to the manufacturer's instructions. Garnish each serving with a twist of lemon zest.
If you do not have an ice cream maker, you may freeze it in a tall container. Freeze for 1 1/2 hours. Remove and stir with a whisk. Return to the freezer and stir about once every hour for about 4 hours. The more times you stir, the more air will be incorporated, resulting in a lighter finished product.