1 tablespoon grated lemon rind
2 teaspoons grated lime rind
3/4 cup lemon juice
3/4 cup lime juice
1 teaspoon citric acid
1 teaspoon tartaric acid
1 1/2 cups Chelsea Caster Sugar
3 cups boiling water

Place lemon rind, lime rind, lemon juice, lime juice, citric acid, tartaric acid and Chelsea Caster Sugar in a heatproof bowl. Add boiling water. Stir until sugar has dissolved. Strain into a heatproof jug. Pour into hot, sterilised bottles and seal. Refrigerate until chilled.
To serve: Use 1/3 cup cordial to 2/3 cup chilled sparkling mineral water or iced water. Serve with thinly sliced lemon and lime.