Place the sugar into a large saucepan/stockpot or a large Pyrex bowl. Pour the boiling water over and stir until all the sugar has dissolved and leave to cool.
Grate the rind of the lemons with a fine grater, add to the sugar water. Slice the lemons into thick slices and add to the water. Add the citric acid and stir, then finally add the flower heads to the water and stir again. Cover with a clean cloth and leave to steep for 48 hours.
Strain through clean fine muslin cloth into a clean bowl. The using a funnel fill sterilized bottles. Seal and store in a cool, dark place (not the refrigerator).
The cordial is delicious diluted with still or fizzy water or used as a flavoring in ice creams or in fruit fools.
Once a bottle is opened store in the refrigerator.