Put the roast in a roasting pan skin side up. Drizzle with olive oil and sprinkle the skin with salt. Place in a preheated 180°C oven and roast for 2.5 hours. Remove from the oven, place on a plate, cover loosely with foil and rest.
Pour the fat from the roasting pan and place it over high heat. Add a big splash of dry cider and a knob of butter, scrape up the caramelised meat juices off the pan with a wooden spoon and boil until slightly reduced.
Serve the pork sliced with the cidery juices on top.