Sorry, but life is too short to go to the pharmacy especially to buy citric acid. But by all means, go ahead and buy it if you, unlike me, are planning to preserve your cordial for a long time. My plan is to drink it all within a few days, hence the small quantities (multiply as necessary). You can always add more lemons as they contain citric acid.
This makes about 250 ml of cordial.
300 ml water
250 gr sugar
50 gr elderflower heads
50 gr nettle
In a pan, heat the water and sugar gently on a medium heat until all the sugar has dissolved (about 5 minutes). Add the lemon, sliced, and the nettle, and let the sugar syrup cool down completely.
Wash the elderflower heads and add them to the sugar, nettle and lemon syrup. Transfer the liquid onto a sterilised bottle or jug and cover with cling film. Keep the liquid in the fridge for 48 hours to let the flavours infuse. Filter the cordial through a sieve covered with a muslin cloth into a sterilised bottle and keep in the fridge or pour into an ice cube tray.
If you don’t want to waste the flowers and nettle you can add them to a smoothie or a cake.