For the pudding, grease a 1.7 litre/3 pint pudding basin with a tight-fitting lid.
Place the breadcrumbs, flour and sugar into a large mixing bowl.
Melt the butter with the marmalade in a pan over a low heat, but do not boil.
Pour the butter and marmalade mixture into the bowl of dry ingredients and mix together thoroughly.
Place the eggs into a bowl and whisk until light, then stir into the marmalade mixture until well combined.
Dissolve the bicarbonate of soda in a tablespoon of cold water. Add to the pudding mixture and stir well. The mixture will increase in volume as it absorbs the bicarbonate of soda.
Transfer the mixture to the prepared pudding basin and leave to stand for five minutes.
Cover with a close fitting lid, or kitchen foil tightly secured with kitchen string.
Carefully place the pudding basin into a pan of boiling water - the water should reach halfway up the side of the pudding basin. Simmer the pudding for two hours. The water level may need to be topped up during the cooking period.
For the Drambuie custard, place the vanilla pod and seeds into a pan.
Add the cream and milk and slowly bring to the boil.
Place the egg yolks and caster sugar into a clean bowl and whisk until pale and creamy.
Gradually pour the boiled cream and milk mixture into the eggs mixture, stirring all the time. Pour the custard back into the pan and add the Drambuie. Place over a very low heat and stir with a wooden spoon for 2-3 minutes, until it begins to thicken.
To serve, uncover the pudding and turn it out onto a serving dish. Melt five tablespoons more of the marmalade in a pan, then drizzle over the pudding. Cut the pudding into wedges and serve with the Drambuie custard poured over.