Preheat the oven to Gas Mark 6, 200°C, fan 180°C. Wash the potatoes then carefully make small vertical slits, 2mm apart, three quarters of the way down each potato, all the way along.
Put the potatoes in a roasting tray. Mix together the butter and oil and brush over each potato. Season well with freshly ground black pepper and salt flakes, and roast for 45 minutes, until golden and crispy around the edges.