For each litre of water allow 10 large elderflower heads, 1 lemon (grated zest and juice) and 1 tsp citric acid (optional) and 1/2 kilo white sugar.
Boil the water and pour over the flowers and zest. Leave overnight in a warm place to infuse.
In the morning strain the liquid off through a scalded piece of muslin placed over a sieve into a saucepan. Add the lemon juice and sugar.
Heat gently, stirring, until the sugar dissolves. then turn up the heat and bring to a boil. Boil rapidly for 3 minutes exactly then take off the gas.
Pour, via a funnel, into sterilised bottles. Seal with a sterilised cap and leave to cool.
Dilute the cordial with water or fizzy water to serve.
I have experimented with lower amounts of sugar but less than 250g/litre and it doesn’t keep well. This gives a less sweet cordial that is not as syrupy.
There is also a recipe for Meadowsweet Cordial and a video