In a very large glass or ceramic bowl, stir together wine, garlic, basil leaves, and rosemary.
With lamb fat side down, puncture meat 5 to 10 times with a long-tined fork. Place lamb, fat side up, in marinade. Cover bowl with plastic wrap. Marinate in refrigerator several hours or overnight, turning meat occasionally.
Lift the meat from the marinade, reserving the marinade. In a covered grill, arrange medium-hot coals around the outside edges of the grill; then, test for medium heat above the center of the grill (not above the coals).
Place meat, fat side up, on a rack in a roasting pan. Sprinkle with pepper. Insert a meat thermometer into the meat. Place pan on the grill rack, but not over the coals. Lower the grill hood. For medium doneness, cook for 2-1/4 to 3-1/4 hours or until meat thermometer registers 145 degrees F. (temperature will rise 5 degree F after it's removed from the grill). Add extra coals as needed to maintain temperature. Baste with reserved marinade.
Transfer meat to a warm carving platter. Cover loosely with foil to keep warm for 10 minutes before carving. Makes 16 to 20 servings.