Preheat the oven to 220˚C, gas mark 7. Place the pork on a wire rack in the sink and pour over 1 litre boiling water; dry well (ideally leave it to dry overnight in the fridge). In a pestle and mortar, crush the fennel seeds and salt, then rub well into the pork skin.
Roast the pork for 2 hours and 15 minutes, turning the oven down to 190˚C, gas mark 5, after 30 minutes. The pork is cooked if the juices run clear when you insert a skewer into the thickest part of the meat. If the crackling is not crisp, place under a hot grill for 5 minutes, then cover with foil and leave to rest for 15 minutes before carving.
Tip the fat from the pan, retaining 1 tbsp or so. Place the pan over a medium heat, add the fl our and stir for 1 minute until the fl our is lightly coloured. Add the Marsala and chicken stock, whisk together, then stir over the heat until thickened – about 5 minutes. Carve the pork and serve with the Marsala gravy.