Snip off the flowers and wash gently (checking for insects!). Shake dry and set aside.
Pour the lemon juice, water and sugar in a heavy bottomed saucepan. Heat gently, stirring all the time until it comes to the boil. Let it boil for 1 minute or so, but not any longer as it will evaporate and thicken too much.
Stir in the clean flowers and set the syrup aside for at least 3 or 4 hours (longer is better) to allow the syrup to absorb the flavour from the elderflowers.
Strain the flowers, then pour into a bottle and seal in the fridge. Use as needed in drinks, icing or drizzled over ice cream.