Break up the potatoes. Heat the oil in a heavy frying pan, then add the potatoes and corned beef and cook, turning the potatoes over with a fish slice every time they become crisp. After about 10 mins, when the potatoes are crisp all over, stir through the beetroot, then season. Turn down the heat, pat the potatoes into a cake, then leave to brown on the bottom. Invert onto a plate, then return to the pan until the other side is browned. Serve straight from the pan, or turn out onto a board and cut into wedges. Serve with the horseradish sauce.