Shake the elderflowers gently to knock off any insects. Washing them removes the pollen, which is where the flavour is.
Add the sugar and citric acid to a large stock pot. Add the hot water and heat gently, stirring occasionally until all the sugar has dissolved. Remove from heat.
Zest the lemons (or use a vegetable peeler to take off strips of peel). Slice up the lemons into rounds. Add the zest or peel, lemon slices and flowers to the sugar syrup. Make sure the flowers are submerged, cover and leave to steep for 24 - 48 hours.
Strain through a cheesecloth or fine mesh sieve, and pour into sterilised bottles. Dilute to taste with chilled still or sparkling water, and serve over lots of ice.
You can also freeze the cordial in ice cube trays, and add the frozen cubes directly to water (or gin, if you're feeling cheeky...)