Mix together the flour, mustard powder and celery salt on a large plate and use to coat the oxtail pieces. Melt half the butter with 1 tablespoon of the oil in a large, flameproof casserole. Brown the oxtail, half at a time, and drain to a plate.
Add the onions and carrots to the pan with the remaining butter and oil. Fry until beginning to brown. Return the oxtail to the pan with the bay leaves and any remaining flour left on the plate.
Mix together the tomato purée, sherry and stock and add to the dish. Bring to the boil, then reduce the heat and cover with a lid.
Place in a preheated oven, 150°C (300°F), Gas Mark 2, for about 3½ hours or until the meat is meltingly tender and falling from the bone. Check the seasoning and serve with plenty of warmed bread.