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.