Roughly chop the potatoes and mash slightly with a fork. Roughly chop the vegetables and mix the two together in a large mixing bowl. Stir in the leftover roast meat, horseradish (or mustard), rosemary and season with salt and milled pepper. Divide the mixture into 4 and form into cake-like shapes. Pop into the fridge for 5-10 minutes.
Heat a large frying pan with the sunflower oil. Toss the cakes into the flour, shaking off any excess and fry for about 5 minutes over a medium heat until golden. Flip and cook on the other side. Serve hot from the pan with either a fried egg or poached egg and a little brown sauce.