Preheat the oven to 180°C / 350°F / Gas mark 4.
Wipe the joint with kitchen paper to remove any excess moisture and place in a roasting tin.
In a bowl, combine the oil, harissa and garlic and rub it over the meat, until it is well covered.
Bring a large pot of water to the boil, cook the potatoes until just tender, drain and in the same pot, swirl them inside it to create a fluffy texture outside though not overly rough.
In a baking dish, add 2 tablespoon of olive oil, some salt and pepper then roast the potatoes until golden and crisp in the last 45 minutes of the pork cooking time.
Roast the pork in the oven for 25 minutes then reduce the temperature to 160°C / 325°F / Gas mark 3.and roast for 2 hours 45 minutes.
Cover the joint in foil and leave to rest for 15 minutes.
Serve the pork with the roasted potatoes and any green vegetable you prefer.