Put the caster sugar and the water in a small saucepan and over a medium-low heat. Heat the pan gently, stirring, until the sugar has completely dissolved.
Bring to the boil and boil steadily for 5 minutes (without stirring), or until the solution has attained the consistency of a thin syrup. Leave to cool, then stir in the lemon juice, and the lemon vodka.
Pour the lemon mixture into an ice cream maker and process following the manufacturer's instructions.
Alternatively, if you don't have an ice cream maker, pour the lemon mixture into a shallow container and freeze for an hour.
Remove from the freezer and beat with an electric mixer until smooth, then return to the freezer.
Repeat this procedure twice to produce a smooth, fine-textured sorbet, then leave to freeze until very firm. Serve in scoops.
