Sprinkle the duck portions with salt and place them on a roasting rack in a roasting tin. Cook in a preheated hot oven at Mk 8 - 450ºF - 230ºC for for 20-25 minutes.
Meanwhile, cut the grapes in half and remove the seeds. Blend the arrowroot with the orange juice in a small pan, bring to the boil and stir in the grapes. Cut the orange into thin slices about one-eighth inch (3mm) thick. When cooked, arrange the duck onto a warmed serving dish and pour the sauce over. Decorate around the serving dish with the orange slices.