Today I found something that might interest you... The book is "Food in England", I'd looked for a recipe for Eccles cakes in it before, but they don't have one. However, under "Plum Pudding Cold Remains (if any; four local methods)" it mentions this:
"Lancashire sensibly uses the pudding, crumbled up with a little extra brandy, to fill a pastry case. This is then turned over, and rolled lightly, and slits cut on top (like an Eccles cake)."
Maybe that's where the idea of Eccles cake with leftovers comes from? Sounds like a good idea! (Although, as they say in the heading... remains - if any!)