To make the stock, in a large saucepan, place the chicken frame, vegetables and salted water. Bring to boil and simmer gently whilst preparing tagine.
Heat oil in a tagine or heavy-based casserole dish, add chicken pieces and sauté, turning once, for 2-3 minutes.
Add Ras el Hanout, black pepper, preserved lemon, olives and water, then reduce heat, cover and simmer slowly for at least 40 minutes, until chicken falls from the bone. Check for salt, but the preserved lemons will have probably made it salty enough.
Place couscous in saucepan or dish and add one cup of the simmering stock. Stir, cover, and set aside for 5 minutes. Fluff with a fork, transfer to a warmed serving bowl.
Using a slotted spoon, lift some of the vegetables from the stock and place on top of the couscous. Serve with the chicken.