Gently wash the elderflower heads and tie the stalks together in a bundle.
Heat the water with the sugar until the water is boiling and the sugar has dissolved, stirring from time to time.
Remove the pan from the heat and add the citric acid. Pour the water into a wide-necked jar, add the lemon slices and insert the heads of the elderflower. Tie the stalks to a spoon and balance it over the neck of the jar.
Let it steep for 24 hours, stirring from time to time, then remove the elderflowers and strain the cordial through a fine sieve.
To serve, dilute the cordial with 5 parts of still or sparkling water.