Halve the plums and discard the stones. Put the plum nalves in a large bowl, scatter the sugar over and leave for 12 hours, covered with a tea towel.
Scrape everything into a metal pot and bring slowly to the boil, stirring often. Boil, uncovered, for 10 minutes. Remove from the heat and allow to cool, then cover. After 12 hours, add the cinnamon stick and then boil again uncovered, for