Cut the marrow into chunks and place in a basin. Sprinkle with the salt and leave for 24 hours. Drain and rinse.
Put the marrow, apples, shallots, sultanas, vinegar and sugar into a large pan. Bruise the ginger, place in a muslin bag with the peppercorns and add this to the pan. Bring to the boil slowly and allow to simmer for 1 to 2 hours until it is of the correct consistency. Stir oc