WINSTON AND THE CHURCHILL DYNASTY - The Uncensored Story, by John Pearson (Pan, $119). ASURE-FIRE method for a would-be serious biographer to shoot himself in the foot, is to slap on a red ''The Uncensored Story'' stamp, setting the scene for a trashy, muck-raking exercise. It is soon obvious John Pearson intends something more serious than the condensed soap-opera of unhappy families into which this biography of Winston Churchill disintegrates. The intention is sound enough: most biographies have laid a heavy emphasis on the military, political and historical aspects of his career while blocking out a perspective of the emotional shipwreck of the Marlborough dynasty which careens through the generations. Of the four Churchill children, only the youngest, Mary Soames, survives and flourishes. Diana, the eldest, killed herself at the age of 54, two years before her father died, while Sarah, who had been Churchill's breezy, rebellious favourite, died an alcoholic at 67, still clinging with infinite pathos to the family wreckage: ''I don't mind going, because I know papa is waiting''. Randolph, though talented, was not born as great as his father who nevertheless thrust greatness upon him. Primed to precocity and arrogance, unashamedly feted and spoiled, he inherited all his father's vices and few of his virtues. Children of great people rarely excel in what made their parents famous: Randolph inevitably underachieved as a politician, and lapsed into the life of a typical Marlborough waster. All three were divorced at least once and died tragically of misfired potential. Churchill was effectively visiting the sins of his own father on his son. Lord Randolph, violently biased against the young Winston for hating Harrow, and refusing to dignify with his concentration any subject that bored him, maths and science especially, condemned him as ''good for nothing, slovenly, extravagant, with little claim to cleverness''. At 37, in the last stages of syphilis, he had literally seen Monte Carlo and died, having wrecked his own brilliant political career with a tactical letter of resignation to the government which was unexpectedly accepted. Ironically, it was Lord Randolph's unintentional reverse psychology which spurred Churchill on to his later successes. In 1897, having distinguished himself at 23 while still a subaltern in the Indian Army with The River War, his account of the war on the North-West frontier, Churchill covered himself in glory during the Boer War by rescuing British troops from a bombarded train. He was taken prisoner and by the time he escaped, he had become a hero. But here Pearson's dubious reliability comes to an abrupt halt, while his axe grinds on inexorably through its hatchet-job of Churchill's life, making way for the reconstruction of Churchill as a power-mad ''effortless dictator'' and ''autocratic monarch'' whose ''elevated'' role in history he rewrites as merely fortuitous. Throughout the rest of the book, inanities describing the design of women's outfits at society weddings rub shoulders with wildly tendentious anti-Churchill statements, accusing him of anti-Semitism, distorting the meaning of his salutary ''blood, sweat and toil'' speech, dismissing as ''hare-brained'' his achievements as a brilliant military strategist, criticising Churchill for stamping on Chamberlain's policy of appeasement and thus, Hitler's Lebensraum. These are nothing compared to the serious inaccuracies with which Pearson attempts to debunk the posterity of Churchill's redeeming leadership in World War II. It is not true that Churchill neglected social issues during the war. But most outrageous is his allegation that Churchill and Hitler were alike. Churchill was, from the accounts of those close to him, a frequently impossible man to live with. He addressed members of his family as if they were a public meeting, planned important domestic moves over the head of his long-suffering wife Clementine, and his house reminded Harold Macmillan of a government department. But he was also deeply emotional, easy to anger and wound and always forgiving, erring on the side of excessive, rather than inadequate love. He was hugely energetic, despite the periodic spells in the grip of what he called ''Black Dog'', his terrible depression that threatened to engulf him especially during his political banishment in the wilderness years. Pearson is the worst type of biographer. Distant without being impartial, he lacks the vital combination of objective perspective and empathy that it takes to convey anything more than the one-dimensional cardboard figure he so incompetently presents. Churchill's big heart, depth of vision, genius and humanity shine far truer through Martin Gilbert's almost purely factual, brilliant study of his career.