Pick the leaves off one rosemary sprig. Grate half the nutmeg and zest both oranges. Cut the oranges in half and set aside for stuffing the duck.
Put everything on a board with one tablespoon of sea salt and chop it all up. Rub the mixture all over the ducks, inside and out. Cover and leave in the fridge for a few hours or overnight to let the flavours penetrate.
Preheat the oven to 180°C/350°F/gas 4 and arrange the shelves on the middle and bottom levels. Stuff the ducks with the remaining rosemary sprigs, garlic cloves and reserved orange halves.
Peel and quarter the onions, then trim and roughly chop the celery and carrots. Peel and chop the ginger. Scatter the veg and ginger in the bottom of a large, deep-sided roasting tray with the cinnamon and bay leaves. Roughly chop the reserved duck neck and giblets and add to the mix.
Place the ducks breast-side up, straight on to the bars of the middle shelf, then pop the veg-filled roasting tray on the bottom shelf beneath the ducks ready to catch all the lovely fat that drips out of them.
Meanwhile, peel the potatoes and cut them into chunks. Pop them in a pan and cover with cold, salted water. Bring to a simmer and parboil for 5 to 10 minutes, then tip into a colander and chuff them up a little.
After the duck has roasted for an hour, take the bottom tray out of the oven, replacing it immediately with an empty tray. Spoon the fat from the veggie tray into a bowl. Put all the veg, duck bits and juices into a large saucepan, then add a little boiling water to the tray to get all the sticky brown bits off the bottom – this is what you’re going to make your gravy with. Tip the water and brown bits into the pan with the veg, top up with 1 litre of water or chicken stock and place on a medium heat, skimming off any of the fat that rises to the top.
Put your parboiled potatoes into the empty tray in the oven. Add a few more tablespoons of duck fat from the bowl, season, and place back underneath the ducks to cook for an hour.
Meanwhile, heat a saucepan and add 2 tablespoons of duck fat. When it’s hot and melted, add the flour and stir with a wooden spoon until you have a paste. Stir in the contents of the saucepan and the Port. Bring the gravy to the boil and simmer gently for half an hour, stirring occasionally. By now the ducks will have had 2 hours in the oven and will be done. Lift them on to a plate, cover loosely with tin foil and leave to rest for about 15 minutes.
Pour the gravy through a sieve into a clean saucepan, pressing down on all the veg and other bits to extract as many flavours and juices as you can. Keep the gravy warm in the saucepan, again skimming off any fat on the surface.
When you're ready to dish up – don’t carve the ducks! The best thing to do is to pull the meat away from the bones with a pair of tongs or with your fingers wearing clean kitchen gloves, then let everyone fight over the delicious skin. Serve with your potatoes and gravy.