Put the flower heads into a large bowl. Add the sugar, the zest of the lemon finely pared, its juice and half the lemon skin, the vinegar and water – you can use either tap or bottled water, as you prefer. Cover loosely and leave for 24 to 36 hours. Strain through fine cheesecloth or muslin and bottle in screw-top or corked bottles. Leave for three weeks before serving, chilled.