Grease a 1.7 litre pudding basin with a tight-fitting lid.
Combine breadcrumbs, flour, and sugar in a large mixing bowl.
Melt butter with marmalade in a pan over low heat without boiling.
Mix the butter and marmalade mixture into the dry ingredients thoroughly.
Whisk eggs until light and stir into the marmalade mixture.
Dissolve bicarbonate of soda in 15ml of cold water and stir into the pudding mixture.
Pour mixture into the basin and let stand for five minutes.
Cover tightly with a lid or foil secured with string.
Simmer in a water bath for two hours, maintaining water level halfway up the basin.
Place vanilla pod and seeds into a pan for the custard.
Add 300ml cream and 300ml milk and bring slowly to a boil.
Whisk egg yolks and caster sugar in a clean bowl until pale.
Gradually whisk the hot milk mixture into the eggs, then cook with Drambuie over low heat until thickened.
Turn out the pudding, drizzle with extra melted marmalade, and serve with custard.