The day before you want to serve, preheat the oven to fan oven 200C/conventional 220C/gas 7. Put the potatoes into a pan of lightly salted water, return to the boil and cook for 5 minutes. Drain the potatoes, put them back into the pan and place it back on the heat for a couple of minutes to dry out.
Meanwhile, pour the oil into a large roasting tin (you may have to use two) and heat it in the oven until smoking hot. Now stir the potatoes into the hot oil and return to the oven to roast, turning occasionally, for 55 minutes.
Cook the swede in boiling salted water for 50-55 minutes, or until very soft. Drain and add to the roasted potatoes. Roughly mash everything together, keeping quite chunky, then cool, cover and keep in a cool place.
To serve, preheat the oven to fan 180C/conventional 200C/gas 6. Uncover the potatoes and swede, dot with the butter and put in the oven to reheat for 25-30 minutes, stirring now and again until piping hot. Serve with lots of butter.