There is no need to remove the currants from their stalks.
Put the currants in a preserving pan with the water and simmer gently for 30 minutes, until the fruit is really soft and pulpy.
Stir from time to time to prevent sticking.
Spoon the fruit pulp into a jelly bag or cloth attached to the legs of an upturned stool, and leave to strain into a large bowl for at least 12 hours.
Do not squeeze. Discard the pulp remaining in the jelly bag.
Measure the extract and return it to the pan with 450g sugar for each 600 ml extract. Heat gently, stirring, until the sugar has dissolved, then boil rapidly for about 15 minutes.
Test for a set and, when the setting point is reached, remove the pan from the heat. Stir in the port, skim the surface with a slotted spoon and pot and cover the jelly.