Cut the marrow into small pieces, put in a bowl and sprinkle liberally with 2 tbsp salt. Cover and leave for 12 hrs.
Rinse and drain the marrow, then place in a preserving pan or large saucepan with the shallots, apples, sultanas, ginger, sugar and vinegar. Tie the peppercorns in muslin (or put into a small enclosed tea strainer) and place in the pan. Bring to the boil, then reduce the heat and simmer, stirring from time to time, until the consistency is thick.
Leave to settle for 10 mins, then spoon into sterilised jars (see tip below), put on the lids and label. Will keep for a year in a cool, dark place.