Sprinkle the yeast granules into a cup of boiled tepid water, cover with clingfilm and leave to stand for about an hour. Grate the potatoes into a fermentation bin then stir in the raisins, sugar and wheat. Bring the water to the boil, pour over the ingredients and stir well until all the sugar has dissolved. Cover and leave to cool. When cool, add the activated yeast, cover loosely and keep in a cool place for 3 weeks, stirring at regular intervals. Strain the liquid into a demi-john topping up to 1 gallon with cold boiled water if required. Fit an airlock and leave to ferment. When fermentation has ceased, siphon the wine into a clean demi-john and add 1 crushed Campden tablet [a sulpher based tablet that kills unwanted yeasts and bacteria]. Bung [not bum] tight and store for a least six months. Finally, siphon into sterilized bottles, seal tightly, and then, before drinking, take care to judge the potency of the brew!