Fresh Horseradish Sauce with Chives

I don’t normally make real horseradish sauce, as I’ve already confessed, but I give you Arabella Boxer’s recipe, which is staggeringly good. Supermarkets have started stocking fresh horseradish root now (and you can stash some in the freezer), but, if you can’t find any, use instead bottled grated horseradish that has been preserved in soya bean oil and citric acid; Arabella Boxer warns again