Put the sugar in a heatproof jug. Pour over the measurement water and stir until the sugar has started to dissolve. Pour in the lemon juice and stir well until all the sugar has dissolved.
Pour the mixture into a shallow freezerproof container and cover with clingfilm. Chill for 30 minutes.
Churn in an ice-cream machine according to the manufacturer's instructions. Alternatively, freeze in the same container, beating the sorbet with a whisk at 45-minute intervals to break up the ice crystals, until almost completely frozen. Process the sorbet in a food processor or blender until smooth. Freeze until solid.
Transfer to the refrigerator 10 minutes before serving to soften slightly. Serve in individual bowls or glasses, decorated with lemon rind curls and accompanied by shortbread biscuits. The sorbet is best eaten on the day it is made.