Pick the leaves off one of the rosemary sprigs and place on a board with the nutmeg, orange zest, thyme and one tablespoon of sea salt. Chop everything together and 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 place the shelves on the middle and bottom levels. Stuff the ducks with the remaining rosemary sprigs and orange halves, and the garlic cloves, then place them breast-side up, straight on to the bars of the middle shelf. Scatter the onion, celery and carrot in the bottom of a large, deep-sided roasting tray with the cinnamon, ginger, bay leaves, and chopped duck neck and giblets. Place on the bottom shelf beneath the ducks so it will catch all the lovely fat that drips out of them.
Meanwhile, place the potatoes in a pan. 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.
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 port gravy.