Preheat the oven to 190°C, fan 170°C, gas 5. Carefully wipe the skin of the pork dry, using kitchen paper, and rub the salt into the scored skin.
Weigh the joint and calculate the cooking time allowing 30 minutes per 500g, plus an extra 30 minutes. Set the joint on a rack over a roasting tin, put in the oven and roast for 30 minutes. Reduce the temperature to 170°C, fan 150°C, gas 3, and continue cooking for the remaining calculated roasting time. Once cooked, remove from the oven and let rest for at least 15 minutes, covered in foil to keep warm.
While the pork is resting, make the gravy. Spoon any excess fat out of the roasting tin then place it on the hob. Sprinkle over the flour and cook, stirring, for 1 minute. Add the cider or vegetable stock and sugar, and bring to the boil, stirring and scraping all the delicious meaty residues from the bottom of the tin. Let the liquid bubble for a minute or so, then strain and serve with the pork – remove the crackling first to make carving easier.
Cook’s tip: moisture is the enemy of crunchy crackling, so keep the skin completely dry. Resist the temptation to oil the skin first – simply rub in a teaspoon of coarse salt – and don’t baste it during roasting.