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. Discard all seeds, skins and stalks left in the sieve. Set the purée aside to cool.
Whip the cream until thick in a large mixing basin, then fold the fruit purée into the cream. Pour into a glass serving dish and chill for several hours before serving. Serve the blackcurrant fool with soft sponge fingers.