Wash the blackcurrants and remove any leaves, but there is no need to strip the berries from the stems. Place the fruit in a large saucepan and add the water. Cover with a lid and simmer over low heat for 10 minutes or until the fruit is soft. Draw the pan off the heat and stir in the sugar. Rub the fruit and juices through a sieve, preferably a nylon sieve, into a mixing basin to make a purée.