Preheat the oven to 350. Using a pastry brush, coat the bottom and sides of a loaf pan with 1 tbsp of the softened butter. Sift the flour, baking powder, cinnamon, and salt into a small bowl and set aside.
In a large bowl, cream the remaining 4 tbsp of butter and sugar together. Add the eggs and mix well. Add the flour mixture, orange peel, vanilla, milk and water and stir until the batter is smooth.
Stirring constantly, melt the marmalade over low heat in a small pan; then pour it into the loaf pan. Pour in the batter, and bake in the middle of the oven for 40 to 50 minutes, or until a toothpick comes out clean.
Cool the pudding in the pan for 10 minutes. Run a knife along the outside edge of the pan and invert onto a serving platter. Serve warm.