1. Preheat covered barbecue to a medium heat.
2. To make marinade, in a bowl mix rosemary, mint, vinegar, oil and black pepper to taste to combine.
3. in the surface of the lamb Cut several deep slits and fill each slit with a slice of garlic.
4. in a glass or ceramic dish place lamb, pour over with marinade, turn to coat, cover and marinate in the refrigerator for 4 hours.
5. on a wire rack set in a roasting tin place lamb and pour over marinade.
6. Place the roasting tin on rack in barbecue covering barbecue with lid and cook, basting occasionally, for 1 Â½ -2 hours until lamb is tender.
7. With cover let it stand for 15 minutes before carving.