Heat the water and sugar in large pan, stirring until the sugar has dissolved, then transfer to a non-metallic bowl. Cool and stir in the citric acid.
Shake any bugs out of the flower heads, (more for the bugs benefit than yours, as you'll be straining the syrup later) before adding the blooms to the sugar syrup, along with the sliced lemons.
Cover and leave in a cool place to steep for 2-3 days. Stir two or three times daily. Done!
Strain through a muslin (or clean kitchen cloth) into sterilised bottles, and store in a cool dark place, or fill plastic bottles (leaving room for expansion) and pop in the freezer for a year long supply.