1) Squeeze the juice from the lemons and limes over a sieve into a glass measuring cup (about 250ml of juice).
2) In the bottom of a tall pitcher, use a wooden spoon to gently muddle the mint leaves with 50g of the sugar. Add the sieved juice; stir well to dissolve the sugar. Add the cold water and the remaining 100g sugar to the pitcher; stir well to dissolve the sugar.
3) Taste for sweetness and add more sugar, if desired. Chill before serving.
4) Serve poured over ice in tall glasses garnished with lemon and lime slices.