Use king Edward or Maris piper potatoes for the fluffiest mashed potatoes...
Pop the peeled and chopped potatoes into a saucepan, add a pinch of salt, cover with water, bring to the boil and cook until tender. Transfer to a colander and drain well, then return to the pan and set over a very low heat for 2 mins to dry completely.
Use a potato ricer and mash your potatoes, if you don't have one of these you can push the potatoes thorough a sieve for the same effect. Pop the mashed potatoes back into the large saucepan and onto a very low heat and stir vigorously with a wooden spoon. Add your unsalted butter bit by bit making sure it is all fully incorporated before adding the next bit, season with salt, to taste, and serve.