Bring the water to boil in a large saucepan. Pour the boiling water over the sugar and allow to dissolve.
Add the lemons and limes to this sugar solution.
Allow the syrup to cool.
Rinse the elderflowers under a cold running tap, ensuring that all the woody stalks are removed so only the flowers remain.
Add the elderflowers and citric acid to the cooled syrup, cover and leave in the fridge to infuse for 24/48hours.
Pass the cordial through a sieve and pour into clean sterilised bottles. Plastic squash bottles are great to use if you wish to freeze some cordial for months to come.
Serve with lots of ice, lime and mint. You can mix with sparkling water, soda or even champagne for a treat!