Mix the raisins, sugar, wheat and grated potato with the hot water. When cooled to just warm, add the yeast. Set aside in a warm place and allow it to ferment for three weeks. Stir it every day. Carefully strain off through a jelly bag into bottles but do not squeeze the bag as this will make the wine cloudy. Seal and store. The wine will clear itself in about four weeks and should be fit to use soon afterwards but it will improve with longer keeping.