Blend the cordial, lemon juice, mint leaves and sugar.
Pour into a large jar and add chilled water to make up the volume to 1 litre. Taste and add more lemon juice/sugar/mint as required.
Pour into a tall glass over ice cubes and garnish with lemon slices and mint sprigs.
Enjoy!
Notes
The measurements are only approximations and you need to keep tasting as you go as the tanginess of the lemons and size of the mint leaves, all make a difference.