Sharpen your knife and make diagonal cuts across the skin of the pork belly, about 1 cm apart, cutting through the skin to the fat below. These cuts will later serve as your guide for carving the pork. Now cut the pork belly in half lengthwise, to make 2 strips, each about 6 cm in width. Mix the flaky salt and sugar and rub all over the meat sides of the pork, then rub the table salt over the skin. Put the pork in a dish or bowl (to catch any juices) in the fridge, uncovered, for up to 24 hours.
Set a small rack over a roasting tray and grease the rack or spray with oil or non-stick spray. Put the pork on the rack (discard any juices) and roast at 130°C/ Fan 110°C/ Gas 1 for 3–3.5 hours. The meat should be very tender when poked with a knife and if you have a digital thermometer, it should read 90°C when inserted into the thickest part of the meat. Remove the pork to a plate, cover with foil and leave for up to 1 hour.
For the apple chutney, heat the oil in a medium saucepan and cook the shallot, apple and ginger gently, stirring often, till beginning to soften (it depends on the apples, but about 5 minutes). Stir in 2 teaspoons of sugar, the cider, spices and raisins (if using) and cook for 4–5 minutes longer, till the mixture is saucy and glistening. Season with salt and extra sugar if necessary and set aside, covered.
To crackle the skin of the pork, put the oil into a small frying pan and add the pork, skin-side down. Raise the heat and fry steadily for about 8 minutes, moving the pork occasionally, until the skin blisters (‘popcorns’) and turns golden and crisp. Leave to rest for 5–10 minutes, then sprinkle the crackling with a little salt and sugar to finish. Remove to a plate and carve (or cube) along the cuts you made previously, and serve with the warm apple chutney.