Boil the whole potatoes in their skins for 30-40 mins, or until tender (timing depends on their size). Drain and peel off the skins. Put the potatoes back in the pan, cover and gently heat for a min or two.
Remove from the heat and mash the potatoes with the butter until no lumps are left, then heat 100ml whipping cream and 3 tbsp milk together and add that to the potatoes. Beat 2-3 tbsp grainy mustard in and season to taste.