The recipe, as ever when it comes to jams and chutneys, is from Marguerite Patten's 500 Recipes - Jams Pickles Chutneys
2 pints vinegar
1.5lbs cooking apples (after peeling and coring), chopped
1lb onions, peeled and chopped
2 tsp ground ginger
3 tsp salt NB I found this too much so start with 1 tsp
1oz pickling spices
1. Simmer damsons in 1 pint vinegar until tender enough to remove the stones.
2. Add chopped apples, onions, ginger, pickling spices (tied in a piece of muslin), and salt
3. Continue cooking until the fruit is completely soft.
4. Add the rest of the vinegar and sugar.
5. Boil steadily until thick (NB take care it does not catch on the bottom of the pan)
6. Put into hot jars and seal down.
Leave for 6-8 weeks to mature.