750ml fresh lemon juice
550g sugar
3/4 tablespoon citric acid
1½ tablespoons tartaric acid
1 tablespoon boiling water

Mix the citric acid and tartaric acid with the boiling water and stir until dissolved.
Put a large saucepan on a low heat and add the lemon juice and sugar. Stir until the sugar is completely dissolved.
Add the citric acid/tartaric acid mix and stir in well.
Strain through a couple of layers of clean muslin cloth into sterilised bottles. Cap with lids or corks and seal.
When opened, store in the fridge.