Mix the oil, lemon rind, lemon juice, wine, pepper and thyme in a jug. Using a sharp knife score the fat cap of the rump in a checker-board fashion. Then using the point of the knife, make a few incisions in the lamb and insert slices of garlic and rosemary. Pour the oil mixture over the lamb and ideally marinate in the refrigerator for three hours or overnight, turning the lamb occasionally.

Half an hour before cooking the lamb remove from the refrigerator, drain and allow the meat to come back to room temperature. Place the meat on a hot barbecue grill and cook for 10 minutes on each side. Allow the meat to rest for 5-10 minutes before carving across the grain and serving.