Jeremy Thorpe enters the Old Bailey central criminal court here each day with his stolid, dark-haired wife, Marion, the former Countess of Harewood, who maintains the upright, dignified bearing of a woman once married to a cousin of the queen.

But Thorpe slouches gloomily under his familiar brown hat. Just five years ago, as the charlsmatic leader of the resurgent Liberal Party, he was Britain's most dashing young politician. Now, ashen and hollow-cheeked, he is a ruined man who looks older than his 50 years and seems to be receding inside himself.

The flamboyant insouciance of his years of surprising political success in the mid-1970s is gone.He even has abandoned the jocular bravado of last autumn when he sprawled defiantly in his Edwardian three-piece suits across the front bench of a provincial magistrate's court to hear, along with the rest of Britian, the crown's case against him. That unusually full and public preliminary hearing ended with Thorpe being ordered to stand trial here on charges of inciting and conspiring to murder a man who claims he was Thorpe's lover.

For the past month, in a lengthy trial that will continue at least through June, Thorpe and Britain have gone through it all again. The prosecution has painstakingly presented - and almost completed - its case against Thorpe in an ornate, oakpaneled courtroom of Old Bailey, the famous Victorian courthouse. The majesty of the setting contrasts markedly with the seamy melodrama of sex, violence and deceit that has unfolded before the jury of nine men and three women.

The star witness once again was tall, angular and nervous Norman Scott, failed social climber, frequent psychiatric patient, sometime male model, itinerant show-horse trainer and financial parasite of a succession of male and female lovers and benefactors. He repeated in the same sniffling maner, often barely audible in the hushed courtroom, his detailed account of being seduced, kept and abandoned by Thorpe, then a young member of Parliament.

Scott, 38, presented himself as a pitiful figure who, his protests aside, achieved through this case an importance he vainly sought all his life. Born Norman Josiffe, he had taken to calling himself Norman Lianche-Josiffe (Lianche being his version of Lynch, his mother's maiden name) because so many acquaintances in the horsy social set he aspired to had hyphenated names. Later, to help his brief modeling career, he called himself Norman Scott.

According to Scott, Thorpe paid Scott's clothing bills at London's smartest shops, took him to dine at exclusive clubs, restaurants and hotels, had him to tea in the House of Commons and provided him a room near Westminster. But Scott complained that Thorpe mocked him both in public and in private, treating him only as a sex object.

"I realized he did not care for me as a friend," Scott said in court.

Scott testified that he became increasingly distraught as the relationship ended, trying to commit suicide with sleeping pills twice and contemplating killing both Thorpe and himself. He also said he burned his own passport in anguish and sought desperately to have Thorpe do something about his lapsed national health insurance so he could work in Britain.

Although he frequently sought money from Thorpe and Thorpe's mother and accepted payoffs from Thorpe's friends, he said it was never his intention to blackmail Thorpe or make the affair public until an attempt was made to kill him.

Perhaps the key witness against Thorpe was slim Peter Bessell, also a former Liberal Party member of Parliament and close friend to Thorpe. Bessell had fled bad business debts in Britain for California.

Bessell again recounted how Thorpe confessed to him about having an affair with Scott, seeking a meeting, was threatening to expose it. This was in 1974 when Thorpe led the Liberal Party to their best election showing in nearly a half century, winning 19 percent of the national vote and 14 seats in Parliament.

Bessell testified that after efforts were made to placate Scott with several small payments and other help, Thorpe plotted with Bessell and the former deputy treasurer of the Liberal Party, David Holmes, in Thorpe's House of Commons office to kill Scott saying that "it's no worse than killing a sick dog." According to Bessell's testimony, Thorpe considered methods of murdering Scott and disposing of his body, including dropping him down an abandoned mine shaft.

Bessell, in return for his testimony, has been given broad immunity from prosecution, including an attempt he testified that he and Thorpe made to defraud the Liberal Party's most generous contributor, millionaire Bahamian businessman Jack Hayward. The money was to be used to pay Bessell's debts of more than half a million dollars.

Bessell testified that he, too, contemplated suicide, and was for five years addicted to a hypnotic sleeping drug, Mandrax, which he said "clouded" his moral judgment.

Cocky charter airplane pilot Andrew Newton described how he agreed to a $20,000 contract to murder Norman Scott and later shot Scott's Great Dane on a dark, windy night in October 1975, on a desolate moor in southwestern England. Newton testified, however, that he could not bring himself to kill Scott, who stood mournfully over his dead dog's body. Newton said he pretended that his gun jammed before driving off.

Newton served a year in prison on a firearms conviction for killing the dog.

Both Newton and a friend, David Miller, a silk-screen printer in Cardiff, Wales, testified that Newton was recruited and paid by the three businessmen also on trial with Thorpe on conspiracy to murder charges: former Liberal Party deputy treasurer Holmes, who was the best man at Thorpe's first wedding; George Deakin, a Welsh slot machine distributor and nightclub owner; and John Le Mesurier, owner of a discount carpet store in Wales.

Under cross examination by defense lawyers, all four prosecution witnesses admitted they have lied frequently about their roles in the affair in the past.They also admitted selling their stories, both true false versions, for substantial sums of money to british newspapers, American television networks, and book publishers before Thorpe was charged and all witnesses were gagged, under British law, until the end of the trial.

Scott charged $100 an interview and charged more for photographers. Bessell set up his own company to market a still-unfinished book and sold the serializaion rights to the Sunday Telegraph newspaper her for $100,000. Newton testified that he received a total of more $22,000 for being interviewed by the London Evening News, West Germany's Der Spiegel, ABC television, Canadian televsion, The London Daily Express and CBS television.

He said a representative of the London Evening News promised him much more at one point, depending on whom he could implicate in the scandal, including "$200,000 for Jeremy Thorpe's head" and $300,000 for Harold Wilson, who was Britain's prime minister when rumors of the Thorpe scandal first circulated in political and journalistic circles here.

Attacks on the credibility of such vulnerable prosecution witnesses will form the foundation of Thorpe's defense. Besides denying the murder charges Thorpe has publicly denied having a homosexual affair with Scott. Thorpe said he only had befriended Scott and tried to help him with his personal problems.

A letter Thorpe wrote to Scott, which Scott later gave to police when he was angry at Thorep, ends: "Bunnies can [and will] go to France. Yours affectionately, Jeremy. I miss you." Scott testified that "Bunny" was Thorpe's nickname for him. There is evidence that other letters from Thorpe to Scott were obtained by Holmes and destroyed.

Lawyers for the other three defendants have contended in their cross-examination of prosecution witnesses that Newton was hired only to scare Scott away from Thorpe and not to kill him. The jury had heard several tape recordings that Bessell, Newton and Miller each made of Telephone conversations with one or the other of the three businessmen defendants allegedly discussing the conspiracy and its cover-up. Miller even hired a photographer to take pictures of Le Mesurier passing $10,000 to Newton in the Welsh countryside, and the photographs were shown to the jury.

All this scandal and intrigue might well have been the stuff of a "trial of the century," as some newspapers billed it in advance, but all of it had already been splashed across Britain's front pages, almost word for word, during the public preliminary hearing last autumn. The drama of this trial has been largely legalistic, as some of Britain's top lawyers on both the prosecution and defense teams clash with each other and such volatile witnesses.

The primary source of suspense, besides the verdict, is whether Thorpe will take the stand in his own defense and risk cross examination by the prosecution.

Thorpe felt it necessary to resign as Liberal Party leader in May 1976, after Scott, at Newton's trial for shooting his Great Dane, blurted out in court that he had an affair with Thorpe. After Thorpe was charged with inciting and conspiring to murder Scott, he stepped down as his party's spokesman on foreign affairs.

Finally, after the prosecution's case was revealed at the preliminary hearing, Thorpe was defeated in last month's national election for the parliamentary seat from North Devon in southwestern England. He had held the seat for 20 years.

The nation saw him on television election night, staring blankly ahead like a man in a trance, as his defeat was announced. His wife, her arm around him, told reporters, "As Jeremy said, politics is that sort of game. You have got to be prepared to win or lose."

No matter what the final outcome, according to William Rees-Mogg, editor of the Times of London and classmate of Thorpe's at Oxford University, the Thorpe's case "will be seen as one of those epic personal tragedies, like that of Oscar Wilde, that will be recounted and examined 100 years from now." CAPTION: Picture 1, A jubilant Jeremy thorpe at 10 Downing Street after 1974 election success . . .AP; Picture 2, . . . weary Thorpe leaves his home for court session at the Old Bailey last month. AP; Picture 3, Jeremy Thorpe's accuser, former model Norman Scott, stands on the spot in western England where he said he was attacked in 1974. Photo taken in November. UPI