Preheat the oven to 210C/410F/Gas 7.
Add the potatoes to a large pan of cold water, bring to the boil and simmer for about 15 minutes, or until almost cooked through. Drain and set aside to steam dry while you prepare the pork.
Bring approximately 500ml/18fl oz of water to the boil. Place the pork joint on a trivet or cooling rack set over a heatproof bowl. Carefully ladle or pour the boiling hot water over the rind of the pork, it should visibly shrink back. Pat the skin dry. Season the joint all over with salt and pepper on the flesh side.
Roughly chop the carrots, celery and onion into quarters and arrange in the bottom of a roasting tray. Drizzle over a little vegetable oil and place the pork on top of the vegetables.
Roast in the hot oven for about 10 minutes, or until the pork is coloured on the outside. Reduce the temperature to 170C/325F/Gas 3 and continue cooking for approximately 40 minutes, or longer if you prefer your meat well done.
For the roast potatoes, pour the vegetable oil into a roasting tray and place over a medium-high heat. Once smoking, add the cooked potatoes, along with all the small bits of broken off potato. Season with salt, then place the tray in the oven. After approximately 10 minutes, turn the potatoes. The potatoes should take about 40 minutes to cook, during this time you will have to turn them a couple of times.
When the pork has cooked, remove it from the oven and carefully cut off the skin and fat in one piece. Wrap the meat tightly in a double layer of foil and leave on a plate in a warm place until ready to carve and eat.
Place the skin on a small roasting tray and return to the oven for a further 10-15 minutes, or until it has become crisp and brittle.
To make the gravy, place the roasting tray that the pork and vegetables were cooked in over a medium heat. Spoon off any excess fat and discard. Add the Calvados and scrape the bottom of the tin with a wooden spoon to remove as much of the caramelised, cooked bits as possible. Add the quince paste and pour in a splash of boiling water. Stir until the quince paste has dissolved.
To remove the vegetables, strain the gravy through a sieve into a clean pan. Really squeeze the veg as the small fibres that pass through will help thicken the gravy. Keep your gravy warm until ready to serve, adding the resting juices from the pork meat just before serving.
Unwrap the meat and carve a chop per person to serve with the potatoes and the gravy.