Crush the blackcurrants with the back of a spoon, then mix with the sugar and mint. Leave covered for 48 hours, stirring occasionally. Drain (saving fruit for a dessert such as a fool), pass syrup through a muslin overnight, then pour into a sterile container and add in the vinegar. Seal and taste after 72 hours. Add more sugar or vinegar to taste. 

Serve with sparkling water or tonic.