Boil the potatoes in a large pan of salted water, with the lid ajar, for about 20 minutes, until tender when you poke them with a knife. Drain well, then tip them back into the pan.
Put the lid on and sit them back on the unlit hob (turn off the heat source) to steam-dry for 5 minutes.
Remove the lid and let the steam clear, then add the margarine and