Preheat the oven to 200°C/Fan 180°C/Gas 6. Cut along the central bone under the chicken and flatten out. Make slits in the breast and legs and place flesh side up in a roasting tin.
Mix the butter, tapenade and preserved lemon. Reserve the zest from the lemon, juice one half and pour over the chicken. Cut the other half into wedges and place under the chicken. Spread the butter mixture over the flesh and bake for 1 hour.
Meanwhile, parboil the potatoes for 5 minutes in boiling water, drain and return to the pan.
Mix the lemon zest, turmeric, ras el hanout and seasoning. Coat the potatoes in the oil and transfer to a roasting tin. Sprinkle with the spiced zest and place in the oven above the chicken for a further 40 minutes, turning once, until the potatoes are golden and the chicken is thoroughly cooked through with no pink meat.