In a pot, add the sugar and water and bring to a boil. Once it comes to a boil, turn it down to a simmer.
Slice the lemons and add to the sugar syrup. Simmer for five minutes.
Take the syrup off the heat and set it aside to cool.
Remove the stems from the elderflower heads and add the flowerheads to the sugar syrup. The syrup still needs to be warm but not hot; if it is too hot wait for it to cool.
Cover the pot with a lid and set aside for 24 hours to allow the elderflower to infuse.
Once the elderflower has infused, remove the lemon slices and flowers and strain the cordial using a muslin cloth.
Transfer the cordial into a sterilised bottle and refrigerate or freeze.
To make a drink, simply pour some cordial into a glass and add soda water or plain water. Adjust the sweetness to your liking by adding more cordial or water.