Heat the vegetable oil in a large casserole until it shimmers, and sear the beef and pork joints on all sides until browned, about 3 minutes per side.
Pour the water over the meat, cover and cook in the preheated oven for 2 hours. Uncover the casserole; remove and discard the bones. With a spoon or spatula, loosen the browned flavour bits from the bottom of the casserole.
Mix together the ketchup, mustard, Worcestershire sauce, lemon juice, salt and pepper in a bowl. Pour the ketchup mixture over the meat, stir to combine with the browned flavour bits, and cover the casserole.
Reduce oven heat to 120 C / Gas 1, and cook for 2 more hours. Shred the meat with 2 forks, pile each bun with about 5 tablespoons of meat and sauce, and garnish with onion and pickles to taste.