Cook the blackcurrants until soft. Add the blackcurrants and 750ml water to a large preserving pan. Bring the mixture gently to the boil, lower it to a simmer, and cook for about 15–20 minutes. Continue until the fruit has softened and is beginning to break down.

Dissolve the sugar over low heat. Add the 1.3kg granulated sugar or jam sugar to the softened fruit. Keep the heat low and stir until the sugar has completely dissolved, taking care that the mixture doesn’t stick.

Bring the jam to a rapid boil. Increase the heat once the sugar has dissolved. Boil the mixture rapidly for about 10 minutes, giving it an occasional stir as it cooks toward setting point.

Check the set on a chilled plate. Spoon a little jam onto the cold plate and wait a few seconds. Gently push it with your finger. If the surface wrinkles, the jam is ready.

Skim and let the jam rest. Take the pan off the heat and remove any foam from the surface. Leave the jam standing for 10 minutes so the fruit can remain more evenly spread through the mixture.

Fill and seal the jars. Carefully pour the hot jam into warm sterilised jars and seal them immediately. Leave the jars undisturbed until they are completely cold, then label them and keep them in a cool, dark place.