Put the potatoes in a pan with plenty of lightly salted water and bring to the boil. Drain immediately and toss with the rosemary.
Transfer the potatoes to a roasting tin with the lamb, garlic, artichokes and peppers. Cover and cook in a preheated oven, 180°C (350°F), Gas Mark 4, for 30—40 minutes or until cooked through and the potato skins are golden. Meanwhile, steam the leeks.
Drain the excess fat and serve the lamb with the roasted vegetables, leeks and any pan juices.