Strip the blackcurrants from their stalks with a fork and put in a saucepan with the sugar and water. Cook gently for 10 minutes or until well softened, stirring often. Remove from the heat and press the berries through a fine sieve to make a smooth purée. You should end up with about 300ml. Stir in the cassis and set aside.
Put the gelatine sheets in a bowl and cover with cold water. S