Dissolve the sugar in the water in a non-aluminium saucepan over gentle heat. Bring to a boil and simmer for 5 minutes. Using a zester, remove all the zest from the lemons, avoiding any white pith. Add the zest and the lemon juice to the syrup and once it starts to boil, take off the heat and stir in the tartaric acid. Strain the syrup to remove the zest and leave to cool completely.

Once cooled, pour the syrup into a 1-litre glass bottle and refrigerate.

To make the lemonade, dilute 1 part syrup with 3 parts soda water. Serve on ice with lots of mint and slices of lemon.

Tip: You can refrigerate the bottled cordial for 1 month. Try the same recipe with limes and infuse your cordial with lemongrass and ginger.