Combine 200g granulated sugar and 240ml hot water in a large jug or heatproof bowl. Stir until the sugar has completely dissolved.
Set the syrup aside to cool while you prepare the lemons.
Roll the lemons on the worktop to loosen the juice, then cut them in half and squeeze to get around 240ml of freshly squeezed lemon juice, discarding any pips.
Pour the lemon juice into the jug with the cooled syrup.
Add 840ml cold water (start with 720ml, taste, then add the remaining 120ml if you prefer a milder flavour) and stir well to combine.
Chill in the fridge for at least 1 hour to allow the flavours to meld.
When you’re ready to serve, fill glasses with ice and pour in the chilled lemonade.
Garnish each glass with a slice of lemon and a sprig of mint, if you like.