Wash the blackcurrants and remove them from the stems. Place into a preserving pan or very large saucepan. Add the water, bring to a simmer and cook for about 10 minutes until the blackcurrants are soft.
Add the sugar and cook over a low heat stirring until the sugar has dissolved.
Bring to a rolling boil and boil for 10 minutes, then test for a set. Continue to boil for another 2 minutes before retesting and repeat if necessary until setting point is reached. If you have a thermometer the mixture should reach 105℃/220°F.
Pour into hot sterilised jars and cover. Label when cool.