Place the raisins, sultanas, currants, almonds, and candied citrus peel in a large bowl. Pour the brandy over the fruit, stir well, cover, and leave overnight so the fruit fully absorbs the liquid.
The next day, combine the breadcrumbs, flour, baking powder, brown sugar, and spices in a bowl and mix until evenly blended.
In a separate bowl, add the soaked fruit mixture along with the grated apple, black treacle, suet, lemon zest, orange zest, and egg. Stir until well combined.
Add the dry ingredients to the fruit mixture and mix thoroughly. The batter will be thick and sticky.
Generously butter a 2-liter (3.5-pint) pudding mold. Spoon the batter into the mold, pressing it down firmly and smoothing the top.
Cut a circle of parchment paper to fit the top of the mold. Lightly butter one side and place it butter-side down on the batter. Tear two larger pieces of parchment paper to cover the top and partially down the sides of the mold. If the batter reaches the top, fold a pleat in the center of the paper to allow room for expansion. Trim any excess paper.
Secure the parchment by folding it down around the sides and tying it tightly with string. A rubber band can help hold it in place while tying. If the mold has a lid, place the parchment on first, then secure the lid.
Bring a large pot of water to a very gentle simmer. Place a folded cloth or trivet in the bottom of the pot to prevent direct contact between the mold and the pot.
Lower the pudding mold into the pot, resting it on the cloth. The water should reach halfway up the sides of the mold. Cover the pot with a lid.
Steam the pudding over very low heat for about 8 hours. The water should steam gently, not boil. Check occasionally and add more water as needed to maintain the halfway level. The longer it steams, the darker the pudding will become.
Carefully remove the mold from the pot and let it rest for 5 minutes. Invert it onto a plate and allow the pudding to release naturally. Let it cool completely.
To store, remove the parchment and wrap the pudding in clean parchment paper, followed by aluminum foil. Wrap tightly and store in a cool, dark place. If refrigerating, wrap tightly in two layers of plastic wrap plus foil to prevent drying. For freezing, follow the same wrapping method and place in a freezer-safe bag.
To reheat, steam the pudding again for 30 to 60 minutes until heated through, or microwave it in a covered, microwave-safe container to retain moisture.
If desired, flame the pudding just before serving and serve with hard sauce (brandy butter), powdered sugar, whipped cream, custard, ice cream, or lemon sauce.