Give the elderflower blooms a good shake to release any creepy crawlies, then gently run them under cool water. Finally, cut off the stalks.
Heat the water in a pan until a rolling boil is achieved then remove from the heat.
Stir in the sugar until it dissolves.
Place the elderflower blossoms in a large bowl and cover with the boiling water.
Add the lemons.
Cover and keep in the fridge for 2-3 days.
Pour the cordial through a fine strainer. You may need to do this several times to get rid of most of the large bits of debris (it is okay to leave a few small specks, though).
Place back in the fridge until ready to use (see note 1)