For those who hadn't come across the phrase ~~ definition of 'plum duff' from Farlex Free Online Dictionary~~

plum duff - a stiff flour pudding steamed or boiled usually and containing e.g. currants and raisins and citron.
--duff pudding - any of various soft sweet desserts thickened usually with flour and baked or boiled or steamed

Often aka 'spotted dick'. Xmas Pudding a specialist form of it.

Lovers of Stevenson's Treasure Island will recall that Captain Smollet disapproved of Squire Trelawney's spoiling of the Hispaniola's crew by such indulgences as ensuring that they were served duff if he chanced to hear that it was any seaman's birthday.

~M~