Put the sugar and 1.5 litre/3lb 5oz water into a large pan. Bring to the boil, stirring, until the sugar has dissolved. Remove from the heat and cool.
Slice the lemons thinly. Put into a large plastic box or a bucket.
Add the elderflower heads to the lemons with the citric acid and the Campden tablets. (CAUTION: Campden tablets produce an irritant gas if inhaled. Keep your face well back from the mixture.) Pour over the cooled sugar syrup. Cover and leave overnight or up to a couple of days.
Sieve and strain through muslin into sterilised bottles and store in the fridge. To serve, dilute to taste with still or sparkling water.