Bring a large pan of water to the boil, add the pork. Return to the boil, skim, reduce the heat and simmer gently, partially covered, for 30 minutes. Drain thoroughly. Using a potato peeler, remove 8 strips of zest from the orange. Put the star anise, cinnamon sticks, cumin seeds, dried chilies and orange zest in a piece of muslin and tie together tightly.
Make the braising liquid by combining all the ingredients in a very large pot and bringing the liquid to a simmer. Add the bag of spices and the blanched pork, and bring back to a simmer, partially covered, skimming all the while.
Cover the pot tightly and continue to simmer gently for 3 hours, until the pork fat and rind are very soft and tender. You can do this up to a day ahead, let the pork cool in the liquid, chill overnight, then reheat gently in the liquid before serving.
Cut the bok choi in halves or quarters depending on their size, or the Chinese leaves into 5cm/2in pieces. When the pork is done, remove it from the liquid. Remove the skin and cut the meat into small pieces.
Add the bok choi to the pot and cook over high heat until it is very tender about 3 minutes. Remove the bok choi from the cooking liquid and place on a warm dish. Arrange the pork on top.
Skim off all the fat from the braising liquid and boil it to reduce it by about half. Pour some over the meat to moisten it and serve the rest separately. (You will probably have a lot leftover.) Serve with rice.