Dissolve sugar in water over a gentle heat, then bring to a simmer. Pour over the elderflower heads with the zest from the lemons - use a potato peeler to get nice wide strips. Slice the rest of the lemons and add to the syrup with citric acid. Stir well, cover and leave for 24 hours in a cool place to infuse. Strain through muslin and pour into sterilised bottles. Once open, refrigerate.