Prick the damsons with a large needle. Put the vinegar, sugar and spice into a pan and stir over low heat until the sugar has dissolved. Add the damsons and cook until soft but not broken. Lift out the damsons with a perforated spoon and put into hot preserving jars. Boil the vinegar syrup for 5 minutes, take out the spices, and pour the liquid over the fruit. Cover tightly.