If you have time to do it, put the pork belly uncovered, skin-side up on a large plate, sprinkle with sea salt flakes and put onto the bottom shelf of the fridge overnight. This will help dry out the skin and give better crackling.
When you are ready to cook, take the pork out of the fridge and leave at room temperature for an hour before it goes in the oven.
Heat the oven to 160C/fan 140C/gas 3. Use a sharp knife to cut down into the potatoes at ½cm intervals but not all the way through. The easiest way to do this is to sit each potato between two wooden spoon handles, which will stop the knife going too far.
Put 2 tbsp olive oil and the butter in a large roasting tin that will fit all the potatoes and put in the oven for 3 minutes to melt and get hot. Add the potatoes, garlic and thyme, and toss in the buttery oil. Sit the potatoes so they are all cut-side up and sprinkle with sea salt flakes.
Pat the pork belly skin with kitchen paper so it’s completely dry, then rub in a little more olive oil and sprinkle with fresh sea salt. Sit on top of the potatoes and pour the wine around. Use foil to cover the potatoes, but leave the pork skin exposed to the heat of the oven.
Cook for 1½ hours then take out the tin and remove the foil. Increase the heat to 220C/fan 200C/gas 7. Baste the potatoes with the pan juices then put the tin back in the oven for 45 minutes-1 hour or until the potatoes are crisp and the pork skin has turned to crackling (turn up the heat for the final 10 minutes to help it crisp up if you need to).
Rest the pork and potatoes for 20 minutes. Mix the gremolata and scatter over before serving.