Sirloin is lovely and tender, best cooked medium, place joint in a roasting dish. Preheat oven to 240C/ Gas 9. Roast for 20 minutes before reducing the heat to 190C/ Gas 5. Then: If you prefer beef rare: cook for 20 minutes per 450g. For medium rare, cook for 25 minutes per 450g, For beef that's well done, cook the beef for 30 minutes per 450g. (It is best to calculate the cooking time first!) Allow the joint to rest for 10 minutes before serving.