Put the blackcurrants and water in a preserving pan or a large, heavy-based saucepan and cook, stirring occasionally, on low heat for 20–30 minutes, or until the fruit is soft and the contents of the pan are considerably reduced. Add the sugar and stir over low heat until it has dissolved completely. Bring to a full rolling boil and boil for 10 minutes, then test the jam for a set. If setting point has not been reached, boil for 5 minutes more, then test the jam again. Stir in the rum, then pot and cover immediately.