Inspect all the elderflower heads and remove any bugs. You can also fill the kitchen sink with water and swirl in the elderflower heads for a few seconds. Trim all large stalks. Set aside, on paper towels to dry.

In a stock pot, combine water, sugar and lemon juice. Bring to simmer and cook until the sugar dissolves. Remove from heat and let cool. Stir in citric acid. Add the elderflower heads and lemon slices. Cover pot with plastic wrap or a tight lid and soak the elderflower at room temperature for 24 to 48 hours.

Line a sieve with a cheesecloth and place over a large bowl. Drain the syrup, squeeze the lemon slices to get the juice and discard the solids. Transfer to sterilised bottles and store in the fridge for up to three months. For longer storage transfer strained syrup to a saucepan and bring to a boil over medium heat, then transfer to sterilised bottles and store in the fridge or a cool dark place.

Serve diluted with water or soda.