Put the sugar in a large pan or bucket (a jam pan is perfect).
Add the boiling water and stir until all the sugar has dissolved.
Grate the zest of the lemons into the pan.
Pour in the freshly squeezed lemon juice and stir.
Shake the elderflower heads free of any insects, and submerge them in the sugar syrup.
Cover with a tea towel and leave to cool and infuse for 24-48 hours. Stir every now and again.
Strain the liquid through muslin (cheesecloth, tea towels or even kitchen paper will work) and funnel into your sterilised bottles.
Store in the fridge for up to six weeks. Serve with chilled sparkling water.