Bring 750 ml of water to a rolling boil in a wide, heavy-bottomed pan.
Tip in 1000 g granulated sugar and 1.5 teaspoon citric acid, maintaining a boil for 5 minutes.
Take the pan off the heat and pass the hot syrup through a fine sieve or clean muslin cloth into a bowl to clarify it, then let it cool to room temperature.
Stir 1250 g lychee juice into the cooled syrup.
Decant the squash into a sterilised bottle, refrigerate for up to 2 weeks, and dilute with water to taste before serving.