Cut off just the slim elderflower stems that hold the blossoms (no thick stems)
Add the blossoms to a large bowl, together with the lemon slices, citric acid and sugar
Pour over the heated water and stir gently but well until the sugar dissolves
Leave covered to steep overnight
The sugar should be completely dissolved and the mixture syrupy, with the flavour of the elderflowers infused into the syrup
Strain the contents of the bowl into a clean bowl through a fine muslin cloth
Decant and store in sterilised bottles in a dark cupboard or the fridge
Once open, store in the fridge
Dilute to your taste with water or sparkling water, (normally 1 part cordial to 4 parts water) - it's ready to drink and will keep for several months
Notes
Smell the blossoms before gathering - you want sweet-smelling ones! Pick the flowers early in the morning, when their scent is stronger. Don't use older blossoms as they'll spoil the taste.