Soak the Fruit: Place the raisins, apricots, mixed peel, glacé cherries and dates in a large glass container. Pour over the brandy and juice and stir until all the fruit is coated. Cover and set aside overnight so the fruit plumps up.
Prepare the Pan: Preheat the oven to 320°F / 160°C (140°C fan). Grease an 8-9" / 21-22 cm round cake pan and line it with parchment paper. The pan should be about 2.75" / 7 cm tall.
Cream the Butter and Sugar: Using an electric beater, beat the butter and dark brown sugar until smooth and creamy. Use high speed. It takes approximately 1 minute.
Add the Oil and Flavorings: Add the oil and molasses and beat until combined. Add the salt, allspice, cinnamon, nutmeg and baking powder and beat until incorporated.
Add the Eggs: Add the eggs one at a time, beating until just incorporated after each one.
Fold In the Flour and Fruit: Stir in the flour. When it's mostly incorporated, stir in the soaked fruit, including all the extra liquid in the bowl, and the walnuts if you're using them.
Bake: Pour the batter into the prepared pan and cover with foil. Bake for 2 1/2 hours. Remove the foil and bake for another 30 to 45 minutes, until a skewer inserted into the middle comes out clean with no batter on it. Check at 30 minutes first.
Cool: Remove from the oven and cool in the pan for 20 minutes, then transfer to a cooling rack. Let it cool completely before serving.