Place the sugar, water and lemon juice in a medium saucepan. Cook, stirring every now and then, over high heat until the sugar dissolves and the syrup comes to the boil.
Reduce the heat to medium-low and simmer for 20 minutes, or until the syrup thickens slightly. Stir in the citric acid. Strain the cordial into a sterilised bottle. The cordial will keep, refrigerated, for up to one month.
To serve, pour the lemon syrup into glasses, top with iced water or soda water, stir well and serve immediately.