Prick the plums with a needle. Boil the vinegar, sugar and spices and pour over the plums. Leave overnight. Drain off the syrup and boil it for ten minutes. Pour over the fruit again and leave for twelve hours. Bring to the boil with the plums. Remove the spices and put the plums into hot preserving jars. Boil up the syrup, pour over the plums, and screw on the tops immediately.