Heat oven to 350 degrees. Grease a 10-inch loaf pan and set aside.
Roughly chop any large chunks of lemon peel in the marmalade. In the bowl of an electric mixer, beat 12 tablespoons of the butter, the sugar, the lemon zest and the orange zest until light and fluffy. Beat in the eggs, one at a time, until incorporated. Beat in 1/3 cup marmalade and the orange juice.
In a separate bowl, whisk together the flour, baking powder and salt. Add the mixed dry ingredients to the wet mixture until just combined.
Scrape the batter into the greased pan. Bake until the cake is golden brown and a toothpick inserted into the center comes out clean, about 55 minutes.
Remove the cake from the oven and allow to cool slightly, about 10 minutes. Remove the cake from the pan and set it right-side up on a drying rack. Meanwhile, in a small saucepan, heat the remaining 1/3 cup marmalade over low heat. When the marmalade is melted (but not burning) whisk in the confectioners' sugar and the remaining 1 tablespoon of butter. Pour the glaze over the top of the cake, allowing some of it to drizzle down the sides. Allow the cake to cool completely before serving.