Gently rinse the elderflower heads under cold water and give a little shake to remove any excess water and dirt.
Cut the flowers from the main woody stalks into a large bowl. Stir the zest and lemon juice into the flowers and set aside.
Pour the water, sugar and honey into a large pan. Stir frequently over a medium heat until all the sugar has dissolved. Do not allow the syrup to boil at this point.
When the sugar has dissolved bring the pan of syrup to the boil, then turn of the heat. Stir in the flower mixture, cover the pan and leave to infuse for 24hrs.
Strain the cordial through a fine sieve. Allow the mixture to drain through, do not be tempted to squeeze the cordial out of the flowers. Discard the flowers and funnel the cordial into steriliised bottles. Keep in the fridge for up to a month.