Preheat the oven to 160C/325F/Gas 3.
Season the pork on both sides with sea salt and freshly ground black pepper and set aside.
Heat the butter in a large frying pan and add the apple pieces. Cook without stirring for a few minutes until the apples start to caramelise. Toss and cook for a few minutes more until caramelised all over. Remove the apple from the pan and set aside.
Add the chorizo to the same pan and fry for 3-4 minutes. Add the onion and gently fry with the chorizo until it is soft and the chorizo is nicely coloured and crumbling. Add half the garlic and half the thyme, and cook for a further 30 seconds.
Drain the chorizo fat away from the mixture in the pan and mix in the caramelised apple. Spread this chorizo mixture onto the cut side of the pork(not the skin side), roll up and tie with string.
Heat the olive oil in a frying pan and brown the pork on all sides. Smear the rest of the garlic and thyme into the crackling, seasoning it with salt and pepper. Place the pork in the oven and cook for about 1 hour 50 minutes-2 hours, basting the joint in its juices from time to time. Twenty minutes before the end of the cooking time, increase the heat to 210C/410F/Gas 6-7, to crisp the crackling.
For the balsamic reduction, heat the balsamic vinegar in a small pan and simmer to reduce to about 50ml/2fl oz. It will have a syrupy consistency.
When the pork is cooked, remove the meat from the pan and rest in a warm place for 15-20 minutes, covered with foil. Add the balsamic reduction to the same pan in which you cooked the pork. Add the chipotle en adobo or paprika, redcurrant jelly and chicken stock. Heat through and season with salt and freshly ground black pepper.
Serve the pork in slightly overlapping slices arranged on a long plate, with a little of the sauce poured over along with the rest of the sauce served in a warm jug.