The whole point of Christmas puddings is that thy are made to be stored for long periods. My Mum would often make them in the autumn for the Christmas of the following year. She reckoned they weren't ready to eat for 12 months. All the sugar and alcohol makes them pretty well indestructible. Same goes for the rum sauce, although as it is on a base of milk/cream it is more risky. If there is no breach in the packaging and the texture is right, it should be fine. However, the bottom line is that we have survived as a species for over 100,000 years without the benefit of use-by dates. We have a magnificent system of smell and taste that lets us know if food is wholesome or not. Sniff it and taste a bit - if it's off it will be immediately obvious. IANAB*, of course, but I would have no problem with the pud subject to the sniff and taste test. The sauce I might bin to be on the safe side.
*Biochemist.