Place the sugar in a pan with 600 ml (1 pint) of the measured water and all the sliced lemons. Bring to the boil, stirring well until all the sugar has dissolved.
Remove from the heat and add all the remaining water. Stir, then set aside to cool completely.
Once cold, roughly crush the lemons, to release all the juice. Strain through a sieve, add the ice cubes, and serve in glasses decorated with slices of lemon.