Scrub the lemons, then grate the zest into a pan. Add the sugar with 550ml water. Bring to the boil, then simmer for 5 mins. Remove from the heat and stir in the citric acid. When cool, add the lemon juice.
Pour into small plastic tumblers and freeze until the mixture is slushy. Stir really well, put a straw in the centre of each tumbler and freeze until solid.