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 minute or two.
Remove from the heat and mash the potatoes with the butter until no lumps are left, then heat 100ml whipping cream and the milk together and add that to the mashed potatoes. Stir in 2 tbsp grated fresh horseradish, ready grated horseradish from a jar, or creamed horseradish (for a slightly milder taste), then season to taste.