Place the lamb on a cutting board, use a sharp knife make a long cut through the flesh to the bone. Remove the bone and cut away the excess fat.
Cut through the thickest part of the meat to open it out as flat as possible. Make several cuts in the lamb with a sharp knife and push silvers of garlic and sprigs of herbs into the slits.
Place the meat in a bowl and pour sherry and oil over the meat.
Chop remaining herbs and scatter over meat. Cover and let marinate in the refrigerator for 30 minutes.
Remove lamb from marinade and season with salt and pepper.
Place on a medium-hot barbecue and cook for 30-35 minutes, turning and basting with reserved marinade. OR tie up lamb and roast on a spit for 1 1/2 hours or until done.
Scrub potatoes and then cut them into thick slices; brush with marinade and place them around the lamb. Cook for 15-20 minutes; turning occassionally or until golden brown.