Wash the plums cut into halves and remove stones, place in a large saucepan with the water.
Bring to the boil and simmer for about 1/2 hour until the plums are well softened. Make sure all of the added water is evaporated off.
Add the Sure-Set and heat stirring until it is dissolved, then bring to the boil and boil vigorously for 4 minutes. (See 'Adding Sure-Set' for details on boiling and timing).
Pot and cover in the usual way; label and date. (See 'Potting and covering' for details).