Using a large washing up bowl or stainless steel saucepan pour the boiling water over the sugar. Stir to dissolve.
Grate the zest from the lemons and then half them and squeeze the juice out. Throw the whole lot (zest, juice and skins) into the bowl with the sugar and hot water.
When cool add the citric acid stirring well.
Now add all the elderflower heads submerging them in the liquid. Cover with a clean cloth and steep for 24 hrs.
Sterlise a piece of muslin or new J Cloth in boiling water and strain the liquid into a clean bowl or jug.
Sterlise bottles/jars by placing in a warm oven for 10 minutes. (140 to 160C will be hot enough)
Pour the cordial into the bottles once they have cooled.