For the marination, season the pork and put it to marinate in the vinegar and wine for at least 2 hours, preferably more.
To roast, preheat the oven to 180°C/350°F/Gas 4. Remove the pork from the marinade, pat dry and season a little more. Put the olive oil in a suitably sized roasting dish or casserole, heat over a medium flame, then add the pork and brown it as uniformly as possible. Add the chine bones and the marinade and transfer to the oven for 15 minutes, making sure that the joint is perched on its bones rather than fat side down.
After this 15 minutes the marinade should have nearly evaporated, so add 500 ml of the milk and the rosemary. It will almost immediately boil. As soon as this subsides a little, baste the meat generously and return to the oven. Turn the oven down to 150°C/300°F/Gas 2, and roast uncovered for a further 1 ¼ hours, basting with the milk as often as you can. If the milk is showing signs of drying up add a little more.
Test the joint for doneness by inserting a cooking fork for 10 seconds then withdrawing it and gingerly touching the part of the tines which were in the centre of the meat to your lips. Cooked meat is hot but not scalding (you hope). If you have any doubts, baste the meat and give another 15 minutes – nobody in their right mind likes rare pork.
To finish, remove the pork from the oven and take it out of the roasting dish. Rest on a serving platter in a warm place (perhaps the switched-off oven). Examine the rather unappetising mess in the roasting tray. If rather dry and brown with blobs of cooked milk, add 500 ml water and boil, stirring vigorously, until a recognisable gravy is formed, then pass through a sieve into a small pan and simmer slowly until needed. If on the other hand there is copious liquid, simply boil everything down to concentrate it and sieve as above. The first option is the more likely: as you have been roasting everything uncovered, so extensive evaporation will have taken place.
To carve, remove the serving plate from the heat and transfer the pork to a carving board. Pour or scrape any juices from the plate into the gravy. Cut the pork into slices between the ribs, one slice with a rib, one slice without, and arrange on the hot serving plate. Pour any juices from the chopping board into the gravy and coat the pork with it. Serve with crackling, if done, and the rest of the gravy in a sauce boat.
This joint is very good with boiled new potatoes and sautéed Swiss chard (or spinach).