Rub lamb with olive oil and season well with salt, pepper and dried herbs. Tie leg of lamb at intervals with string to secure. Place in a roasting pan with rosemary and garlic cloves. Roast lamb, basting with pan juices for 20 minutes per 500g and, if necessary, extra 20 minutes at the end of cooking time. Allow to rest for at least 10 minutes before carving. Serve drizzled with pan juices.