Put the lemon slices in a large heatproof jug or bowl, and add the citric acid. Tip in the caster sugar, then pour in the boiling water. Stir to combine and dissolve the sugar. Leave to cool, then sieve and chill.
To serve, pour some of the lemonade into a large jug with ice and some slices of fresh lemon, dilute with cold water or soda water to taste.