Gently shake the elderflower heads to remove unwanted bugs. Collect them in a large bowl or tall jar. Remove the zest from the lemons with a peeler and add it to the elderflowers. Slice the lemons and add them to the bowl as well.
Collect the sugar and the water in a saucepan and bring to a boil, stirring to dissolve the sugar. When the syrup comes to a boil, pour over the elderflowers and lemons. Stir to mix. Cover and leave in a cool, dark place for a day.
The day after, strain trough a muslin-lined sieve and bottle into warm sterilized jars.
You should keep this in the fridge and use within five weeks, even though mine, last year, lasted for longer.