Question: how long does it take to cook 1.2 kg of beef?
Answer: Preheat the oven to 200°C, fan 180°C, gas mark 6. Place the beef on a rack in a roasting tin. Rub the oil, mustard and pepper over the joint. Cook the beef for 20 minutes per 500g, plus 20 minutes for rare (or for medium: 25 minutes per 500g, plus 25 minutes; for well done: 30 minutes per 500g, plus 30 minutes).