mash? I thought the potatoes were the horrible fluffed up english potatoes roasted in goose fat. But mash? That just seems like a way of upping the calorie and glycemic values of an already enormous meal.
(yeah, I've never had a 20th/21st century home cooked British Christmas dinner)
Frittata is another good option, chop everything, fry it up and pour eggs over it.
I'd suggest instead of potato cakes, opening a can of tuna, salmon or crab meat and making fish cakes. Serve with sweet chilli sauce.
When you are sick of leftovers, chop and freeze whatever is left, and enjoy it in a months time.