Put the caster sugar in a pan with 500ml cold water. Bring to the boil, stirring a couple of times until the sugar has dissolved then leave to boil for 5 minutes. Remove from the heat and leave to cool. Stir in the lemon juice and limoncello.
Pour into a wide shallow dish and freeze for 1 hour until ice crystals start to form. Remove from the freezer and scrape the frozen bits back into the mixture.
Return to the freezer and repeat every 30 minutes or so until the granita is frozen, with a fluffy, crystalline texture.