Strip the mint leaves from the stems. Wash the leaves and shake or squeeze dry in a towel. Chop the mint very finely and put in a bowl.
Measure the vinegar and sugar into a saucepan. Stir over low heat to dissolve the sugar. Bring to the boil, then draw off the heat and pour over the mint. Leave until cold. Pour into one or more clean screw-topped bottles. Cover tightly and store away from the light.
To serve, spoon out the amount required and stir in more vinegar to taste.