Heat the oven to 200°C/gas 6. Grease a roasting tin with half of the butter. Add the chopped vegetables and herbs. Rub the duck with seasoning and place on top of the vegetables. Cover with foil and cook in the oven for 1 hour, then remove the foil and cook for a further 30 minutes, until the duck is golden.
While the duck is cooking, place the giblets in a small saucepan of water. Bring to the boil, then cover and simmer for 30 minutes.
Zest and juice 2 oranges and the lemon.
Set aside the zest and juice. Cut the remaining oranges into wedges, then grease a roasting pan with the remaining butter and add the wedges. Roast in the oven for 20 minutes before serving.
When the duck is cooked, remove it from the roasting pan and keep it warm. Place the roasting pan directly on the hob and cook for 5 minutes to add extra colour to the vegetables. Carefully drain off the fat (reserve it for another use), without disturbing the sediment. Add about 400 ml giblet stock, bring to the boil and simmer for 5 minutes. Remove from the heat, mix the arrowroot with a little water and add to the roasting pan. Stir well to thicken the sauce, then pass through a fine sieve into a saucepan. Add the Grand Marnier, sugar, vinegar, and the zest and juice you set aside earlier, and warm over a low heat, but do not allow to boil. Transfer the duck to a carving plate with the roasted orange wedges and serve with the sauce.