Fluffiest roast potatoes

My gran always used to make effortless and perfect roast potatoes. At the time I never questioned why they were so good, but now I know it was down to two things. First was her choice of potatoes - floury not waxy is best for roast potatoes, so buy varieties such as King Edwards, Maris Piper or Desiree. Second was her method of cooking them. If you want to make perfect fluffy roast potatoes, pre-boil them to the point where they're nearly cooked, drain them, return them to the pan, put the lid on and give them a good shake so that the edges get all roughed up. Then put them in hot beef dripping, goose or duck fat or lard which has been heated up in an oven pre-heated to 200C/gas mark 6. Cook for about an hour or until crisp. You're guaranteed golden potatoes with the flakiest, crispiest exteriors and memorably fluffy insides.