Heat the oven to 200C/fan 180C/gas 6. Mash together the ’nduja, garlic, fennel seeds and rosemary in a bowl. Take the pork from the fridge and pat dry with kitchen paper. Put skin-side down on a board, then spread the ’nduja paste evenly over the central part of the meat. Roll up the meat tightly, tie a couple of pieces of kitchen string around the porchetta to hold it in shape, then go down the length, tying reasonably tightly with string every 5-7cm to secure the whole piece in a neat shape. Put into a large roasting tin, join side down. Rub the vegetable oil over the skin and season generously, rubbing all over and into the scores. Roast for 45 minutes before turning the oven down to 160C/fan 140C/gas 3 and cooking for 3 hours. For perfect crackling, turn the heat back up to 200C/fan 180C/gas 6 and cook for a further 30 minutes or until the crackling is crisp.