In small bowl, put rosemary, mint, vinegar, oil and black pepper. Mix to combine. Set aside.
Cut several deep slits in surface of lamb. Fill each slit with slice of garlic. Put lamb in large glass dish. pour over marmalade, turn to coat, cover and refrigerate for 4 hours.
Preheat hooded barbecue to medium. A kettle/weber can also be used. Put lamb on wire cake rack set in roasting pan and pour marinade over. Put roasting pan on rack in barbecue, cover barbecue with lid and cook, basting occasionally for 1 1/2 to 2 hours or til lamb is tender. Cover and let stand for 15 minutes before carving.