Preheat oven to 140°C (275°F). Place the lamb, vinegar, stock, rosemary, garlic and sugar in a deep baking dish and cover with a tight-fitting lid. Bake for 3 hours. Turn the lamb and bake for a further 2 hours. Remove the lid, increase temperature to 180°C (350°F) and bake for 30 minutes or until the lamb has browned. Remove lamb from the pan and keep warm. Pour the pan juices over a large bowl of ice and quickly remove the ice and the solidified fat with a slotted spoon. Place the pan juices into a small saucepan and bring to the boil. Serve the lamb with crispy roasted potatoes, steamed greens and the pan juices.