In a medium saucepan, combine sugar, mint leaves, vinegar and water. Bring to a boil on high, stirring until sugar dissolves. Reduce the heat and simmer for 10 minutes.
Stir the dissolved gelatine through.
Strain the liquid, discarding mint leaves and pour the hot mixture into sterilised jars.
Allow to cool until thickening, then stir extra fresh chopped mint through. Seal well and chill until required.