Put the dates and the water in a saucepan and bring to the boil. Remove from the heat and add the carb soda - it will froth up and soften the dates. Mash the dates to break them up with a potato masher, and mix until a mushy consistency and no solid lumps of dates remain.
Cream the butter and sugar until light and fluffy. Add the zest, then add the eggs one at a time, beating between each egg. Add the flour and date mixture in alternate batches and fold in after each addition. Grease one large and one small steamer and pour in the mixture - don't fill higher than 2/3. Cover each with a double layer of buttered foil and clamp steamer lid tightly over foil cover. Steam in a couple of large covered saucepans in boiling water (water level about 1" up the side of the steamer) for about an hour for the small one, an hour and a half for the large one (test with skewer). Turn out of the steamer onto a serving plate when it has cooled a little.
Put the ingredients for the butterscotch sauce into a small saucepan, and heat slowly while stirring until sugar has dissolved and the butter has melted - do not boil
Serve warm, pouring the warm butterscotch sauce directly over each serve of pudding before adding cream/ice-cream.