Chop raisins and mixed peel. Mix all fruits together and pour over rum. Set aside and soak at least over night and up to a week to enable fruit to absorb the rum.
Cream butter and dark brown soft sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in prepared fruit. Sieve flour, salt, mixed spice, bicarbonate of soda and nutmeg; fold into mixture with breadcrumbs. Mix well.
Line a pudding bowl with several sheets of muslin cloth. Transfer the batter into the pudding bowl and secure with a piece of string.
Place into rapidly boiling water and boil steadily for 6 hours, replacing boiling water as necessary. To stop pudding catching on bottom of pot, place a small plate into boiling water or hang from wooden spoon across top of pot and ensure water bubbles at least 3/4 of way up pudding. Remove from boiling water, hang in cool place to store or refrigerate.
Can be made up to six weeks in advance. On day of serving steam further 2 hours.
Steaming the pudding
This mixture can be steamed instead of boiled. Fill into a well-greased pudding bowl to within 5cm of top. Cover securely with 2 layers of greased greaseproof paper and one of sheet of foil. Secure firmly with string. Place in steamer or large saucepan with sufficient boiling water to come halfway up basin. Water should boil gently, yet steadily; replenish with more boiling water when necessary. The lid of steamer or saucepan should fit securely.
Re-steam 2 hours on day of serving.