Place the caster sugar and water in a small saucepan and stir over medium heat without boiling. Continue until all the sugar has dissolved.
Bring the mixture to the boil then reduce the heat slightly and bubble for 1 minute. Take the syrup off the heat and set aside to cool.
Meanwhile, squeeze lemons and place juice in a large glass pitcher. Top with the extra water to dilute and add in the cooled sugar syrup. Stir to combine.
Chill the lemonade in the refrigerator for 2 hours. Serve with ice and garnish with a lemon slice or two.