Wash the elderflowers, and then place in a large bowl.
Pour the sugar into a pan along with the water and bring to the boil, stirring until the sugar has completely dissolved.
While the sugar syrup is heating, pare the zest of the lemons off in wide strips and pop into the bowl with the elderflowers. Slice the lemons and add the slices to the bowl. Pour over the boiling syrup, and then stir in the citric acid. Cover with a cloth and then leave at room temperature for 24 hours.
Next day, strain the cordial through a sieve lined with muslin, and pour into glasses or jam jars. Screw on the lids and pop into the cupboard ready to use.