Unwrap the pork and place it in the fridge for a few hours. This step is optional but dries out the skin to improve your chances of a crisp crackling.
Preheat oven to 180C (or 160C if you have fan bake).
Score the pork skin. Rub in the oil and the sea salt flakes, pressing it in to make sure it gets into the skin. Place pork, skin side down, in a large roasting pan. Bake for 1 1/2 hrs.
After 1.5 hours, turn the pork skin side up. Turn the oven up to 220C (or 200C fan-forced). Bake for 45 mins, or until the skin is crisp and how you like it. Remove from the oven and rest the pork for 10 mins.