Trim up the duck of any excess fat around the cavities and season inside and out generously with salt and pepper.
Tie the legs of the duck at the 'ankle'.
Place a roasting tin on the lowest shelf of the oven to catch the dripping fat from the duck. Then place the duck in the oven directly on a cooling grid and not in a roasting tin, which allows the duck to cook evenly.Cook on this high heat for 20 minutes.
Whilst this is happening, heat a griddle pan over a high heat.
Slice the potatoes into 1cm (just under ½") thick slices. Pour the oil over the potatoes and season with salt and pepper.
Grill the potatoes for a couple of minutes on each side to get some nice grill marks.
Once the potatoes have the griddle marks and your duck has been cooking for 20 minutes, arrange the potatoes in a single layer on the tray under the duck.Reduce the heat to 180°C or 350°F and cook for 40 minutes.
Roughly chop the onion and celery for the gravy.
Place the giblets from the bird in a 20cm or 8" saucepan with the onion, celery and water. Bring to a boil, then reduce to a simmer and cook gently for the amount of time it takes to cook the duck.
Remove the duck from the oven and tent it with foil, and rest for 20 minutes. This is THE most important part of this recipe!
Shift the potatoes to the middle of the oven and baste them with the juices they are sitting in, and allow them to cook for the 20 minutes of resting time.
Heat a 25cm or 10" frying pan over a high heat and add the red wine for the gravy, and reduce by two-thirds.
Drain off the stock created from the giblets and add it to the wine reduction along with a generous pinch of salt.
Reduce by half again, skimming off any foam.
Add the duck resting juices and whisk in the cold butter just before serving, being sure to taste and season as required!