1. Preheat oven to 180°C. Combine the vinegar, half the sugar, oil, salt and pepper in a bowl. Place the meat in a dish large enough to lie the leg flat and suitable to marinate in. Pour the marinate over the leg and massage into the meat with your hands. Set aside to marinate for 30+ minutes.
2. Place the lamb, skin-side up, in a large baking dish. Add the unpeeled garlic cloves, rosemary sprigs, marinade and stock. Sprinkle the lamb with the remaining sugar, cover with aluminium foil and roast for 2.5 hours. Remove the foil and roast for a further 30 minutes or until the lamb is cooked to your liking.
3. Serve with the roasted garlic.