Grease a 4-cup ceramic or glass pudding mold with 1 1/2 tablespoons of the butter. Line the bottom of the mold with parchment paper, then grease with 1/2 tablespoons of the butter and set aside. Grease one side of a 1-foot length of heavy foil with 1 tablespoons of the butter and set aside. Sift together flour and baking soda in a small bowl and set aside.
Beat the sugar and remaining 11 tablespoons of the butter in a medium mixing bowl with an electric mixer on medium speed until light and fluffy, 3–5 minutes. Beat in the egg, then the milk. Reduce speed, add the reserved flour, and beat, scraping the sides of the bowl with a rubber spatula, until mixture is just combined. Stir in apricots, then 1/2 cup of the marmalade until well combined.
Transfer batter to the prepared mold, cover tightly with the buttered foil, and place on a rack in the bottom of a large deep pot. Add enough hot water to come halfway up the sides of the mold. Cover, bring water to a gentle simmer over medium-low heat, and steam pudding until a toothpick inserted in the center comes out clean, 2–2 1/2 hours.
Remove pudding from simmering water, discard foil, and set aside to cool for 15 minutes.
Meanwhile, heat remaining 1/4 cup of the marmalade in a small saucepan over medium-low heat until syrupy, 1–2 minutes. Invert mold onto a serving plate, unmold pudding, then discard parchment. Spoon heated marmalade over top of pudding.
Serve pudding warm, with custard sauce, if you like.