Bring the water to the boil, scald chopped peppermint leaves with it and let it steep for 10 minutes. Then briefly bring the brew to the boil with fresh leaves.
Sieve the stock through a fine cloth, measure out 1 1/4 liters and mix in a saucepan with the sugar and citric acid. Heat with constant stirring. Just before the liquid boils, add the gelling agent. Stir, bring to the boil.
Pour jelly hot into clean jars and close with the screw lids. Store cool.