Use juicy lemons and limes with pristine skins to get the maximum flavour. The syrup will keep for several weeks in the fridge.
3 large limes
3 large lemons
4 cups caster sugar
1 tablespoon citric acid
4 cups boiling water
chilled sparkling mineral or soda water
sliced lime and lemon
Gently roll the fruit on the bench to release the juices. Halve and squeeze the juice into a bowl. Cover and refrigerate.
Cut the squeezed fruit into 1 cm thick slices. Place in a large bowl with the sugar, citric acid and the boiling water, stirring to dissolve the sugar.
Cover and leave in a cool place for 24 hours.
Strain the syrup into a bowl, pressing on the fruit to extract as much liquid as possible. Discard the fruit. Add the reserved juice to the syrup and strain again into sterilised bottles. Seal and refrigerate.
To serve: If using vodka, put a measure in each glass, add ice and pour over equal quantities of syrup and mineral or soda water. Top with mint and a small slice of lime and lemon.Makes about 5 cups of syrup.