Put the prepared redcurrants into a preserving pan or saucepan with the water. Cook until soft, about 30 minutes. Put into a jelly bag and leave to strain overnight. Measure the liquid, transfer to a preserving pan and add 1 lb. sugar to every pint of liquid. Dissolve the sugar over a low heat, then bring to the boil and boil rapidly until setting point is reached, about 10 minutes. Skim, pour into hot sterilized jars and cover tightly.
Cranberry jelly can be made in the same way, using the same proportions, or very slightly less water.