Preheat the oven to 170 C, Fan 160 C Gas 3 1/2. Prick the skin of the fatty parts of the duck and remove any solid fat in the cavity ( this can be reserved for roasting potatoes).
Put the wing tips in a roasting tin and set a rack or scrunched up foil on top. Rub the duck with oil and sprinkle with sea salt. Lay breast side down on the rack.
Chop three prunes and stuff into the cavity with the apple, half the onions and a few sage leaves. Roast for 2 hours then pour off the fat and turn the duck. Return to the oven for 30 minutes then blast at 190 C Fan 180 gas 5 for 30 minutes until golden.
Meanwhile heat a dash of oil and soften the remaining chopped onion for 10 minutes. Add the chestnuts and cook for 5 minutes. Add the brandy and reduce, then the stock and remaining prunes. Simmer for 10 minutes and set aside.
Let the duck rest, covered loosely with foil, for up to 30 minutes. Squash and squeeze the juices from the cavity into the brandy mixture with the back of a spoon.
Pour the fat from the roasting tin, discard the wing tips and place over the heat. Add the brandy mixture and simmer, scraping the pan as you go.
Chop the duck into serving pieces and serve with mash and spinach.