Heat your oven to 240°C/Fan 220°C/Gas 9. Unwrap the pork and pat it dry with kitchen paper. Set it in a roasting tin, skin side up, and set aside for up to 1 hr to get it up to room temperature.
Tip the black peppercorns into a pestle and mortar and grind them as finely as you can. No pestle and mortar? Use a small bowl and a jam jar or the end of the rolling pin to roughly crush them.
Add 2 tsp China 5 spice and 1 tbsp cider vinegar to the peppercorns. Peel and grate in the ginger. Add 2 tbsp olive oil, a good pinch of salt and stir to make a paste. Massage the paste all over the pork, making sure to rub it into the slits on the skin and in any gaps at the ends of the rolled pork.
Slide the pork into the oven and roast for 30 mins. Baste the skin halfway through.
Remove the pork from the oven and baste the skin well. Lower the oven temperature to 160°C/ Fan 140°C/Gas 3. Return the pork to the oven and gently roast for 2 hrs.
Check the pork is cooked by piercing it with a metal skewer. The juices should run clear. If not, pop it back in the oven till they do.
Transfer the pork to a plate or board to rest, covered loosely with foil, for at least 30 mins before carving. Serve in thick slices with the crackling.