Put the ginger, lime zest and caster sugar in a pan with 600ml (1pt) water. Bring to the boil, then reduce the heat and simmer for 20 minutes, until syrupy. Stir in the lime juice, then strain the liquid through a sieve into a sterilized bottle and leave to cool. Chill until needed.
To serve, pour a little cordial into a glass, then add a handful of ice cubes, a sprig of mint and top up with sparkling water.
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