1. Peel and dice the marrow; place in a large pan.
2. Peel and chop the onions and apples and place in the pan with the dates, sultanas and ginger.
3. Place the pickling spice in muslin and tie up carefully; place in the pan with the remaining ingredients.
4. Cover and heat gently, stirring to dissolve the sugar; then simmer gently for 1 1/2 hours. Remove the muslin bag.
5. Allow to cool before bottling in clean jars, seal and label.