1. To make marinade, place rosemary, mint, vinegar, oil and black pepper to taste in a bowl and mix to combine.
2. Cut several deep slits in the surface of the lamb. Fill each slit with a slice of garlic. Place lamb in a glass or ceramic dish, pour over marinade, turn to coat, cover and marinate in the refrigerator for 4 hours.
3. Preheat covered barbecue to a medium heat. Place lamb on a wire rack set in a roasting tin and pour over marinade. Place roasting tin on rack in barbecue, cover barbecue with lid and cook, basting occasionally, for 1 1/2-2 hours or until lamb is tender. Cover and stand for 15 minutes before carving.