Tip the water and sugar into a large pan and bring gently to a simmer. Stir until the sugar has completely dissolved and then remove from the heat. Leave to stand for 15 minutes, so that the liquid is warm but not boiling hot.
Now, add the elderflowers, lemon juice, squeezed lemons and tartaric/citric acid. Stir everything together well, cover the pan and keep it in the fridge for 24-48 hours to let the flavours mingle.
When you are ready to decant place some muslin cloth into a sieve over a clean pan and pour the liquid through. Bunch up the cloth and squeeze out as much liquid as you can at the end, to catch every last drop of flavour.
Pour the cordial into sterile bottles and store somewhere cool until needed – it should keep happily for about 6 months. Keep in the fridge after opening. Dilute with cold water and ice to your taste. A slice of lemon or strawberry, ribbon of cucumber, or sprig of mint all make splendid garnishes.
