Soak the cherries, mango, cranberries, currants and candied citron in 4 tablespoons rum for at least 24 hours. Cover tightly and store at room temperature.
Preheat oven to 170 degrees C. Butter a 20cm round tin and line with baking paper.
In a large bowl cream butter and brown sugar until fluffy. Beat in the egg. In a separate bowl whisk together flour, bicarb soda, salt and cinnamon, then mix into the butter and sugar in three batches, alternating with treacle and milk. Stir in soaked fruit and diced pecans. Transfer batter to prepared tin.
Bake 40 to 45 minutes. Cool in the tin for 10 minutes then sprinkle with 2 tablespoons rum.
Cut out one piece of baking paper and one piece muslin or cheesecloth, each large enough to wrap around the cake. Moisten muslin with 1 tablespoon rum. Arrange muslin on top of baking paper and turn the cake out onto it. Sprinkle top and sides of cake with remaining rum. Wrap the muslin closely around the cake then wrap with baking paper. Place in an airtight tin and age for at least 10 weeks. If storing longer, douse with additional rum for every 10 weeks of storage.