Place the pork skin-side up on a rack over a roasting tin. Place the garlic and ginger in a pestle and mortar or food processor and pound or process until you get a rough paste then mix in the oil and vinegar. Rub the paste all over the scored skin of the pork. Place in the preheated oven and cook for 30 minutes.
Remove the pork from the oven, reduce the temperature to 150C/300F/Gas 2. Turn the pork over with the skin side down on the rack and return to the oven and cook for 4-5 hours.
Remove from the oven and turn up to the highest setting 220C/425F/Gas 7. Turn the pork over to the crackling side on the rack and roast in the hot oven for the final 20 minutes to crisp up the crackling. Leave to stand for 10-15 minutes before carving.
To serve, cut away the crackling with a sharp knife and break it up into pieces then carve the meat. It should be very tender and succulent.