In a wok or frypan, heat oil over medium-high heat. Add beef strips 1/3 at a time. Stir-fry for 3 minutes or until browned, removing the done pieces to a plate lined with paper towels.
Return all the beef to the wok. Stir in orange peel, garlic and ginger; stir-fry 1 minute.
In a medium bowl, combine cornflour, stock, soy sauce, sherry, marmalade and chilli flakes. Pour this mixture into the beef, stirring constantly. Bring to a boil over medium heat and cook for 1 minute. Serve hot.