In a small bowl, mix the treacle, vinegar, paprika, cumin, mustard powder and chili powder until smooth.
Using a sharp knife, make deep slashes in the skin of the pork shoulder to cut through the skin and fat layer, but not the meat.
Place the joint, skin-side down, in a large dish and rub the spice paste into the meat (not the fat).
Turn skin-side, cover tightly with cling film and put in the fridge overnight, or for 24 hrs to give the pork maximum flavour.
Heat oven to 150C/130C fan/gas 2. Transfer the pork, skin-side up, to a deep roasting tin, rub 1 tbsp sea salt onto the skin and pour 500ml water into the bottom of the roasting tin. Cover tightly with foil and roast in the oven for 5 hrs.
Remove the foil, turn up the oven to 200C/180C fan/gas 6 and cook for a further 1 hr 30 mins or until the pork is very tender and the skin has turned to crispy crackling.
Once the pork is ready, take it out of the roasting tin, cover with foil and leave to rest.
Pour the juices from the roasting tin into a jug and leave to separate. Pour off the fat layer and transfer the remaining juices to a large sauté pan.
Simmer over a high heat, stirring, until reduced to a rich gravy.
Once rested, cut the pork into pieces – it should pull apart with very little effort – and break the crackling into shards.