As the BBC TV dramatisation of the Thorpe scandal reaches its denouement this Sunday, millions will ask: why was Jeremy Thorpe acquitted of conspiring to kill Norman Scott? Because the judge was biased in his favour; because of the skill of his barrister, George Carman; or because he was innocent?

None of the above. He was acquitted because of the dirtiest deal in media history, made by a rightwing newspaper. The significance of this deal was revealed by a jury member in the New Statesman, which was then itself prosecuted by the Thatcher government for contempt of court. The magazine’s victory for free speech was reversed when that government passed a law that today remains the only legacy of the Thorpe trial: creating the very British crime of refusing to cover up wrongdoing, at least when it takes place in the jury room. That, along with the verve and panache of the TV show, should now be central to our debate.

After the acquittal came the questions “how?” and “why?” Twelve people knew the answer, and one was so dismayed at the ineptitude of the prosecution that he wanted to provide it – free of charge, to the Guardian. Reporters David Leigh and Peter Chippindale were told by a juror that all 12 were agreed there had been a conspiracy to intimidate Scott; yet they were angry that they could not convict because this crime had not been charged. Whether it was a conspiracy to murder depended on the evidence of Thorpe’s parliamentary colleague, Peter Bessell, and the judge had told them they could not rely on his evidence because of a deal he’d struck with the Sunday Telegraph, whereby he would be paid double by the newspaper (£50,000, equivalent to £250,000 today), if Thorpe was found guilty. The jury were annoyed, because they felt they had been obliged to acquit guilty men; they wanted to expose the incompetence of the director of public prosecutions and demand that parliament outlaw deals with witnesses of the kind the newspaper had struck with Bessell.

It was a scoop, but the Guardian’s editor, Peter Preston, had moral qualms about publishing it. The system of jury trial had determined Thorpe not guilty and it would not be seemly for the Guardian to spoil the acquittal by revealing what the jury really thought. Preston recognised that other editors might think differently, and over at the New Statesman, editor Bruce Page thought that the public interest in publishing the secrets of the Thorpe jury room was overwhelming.

He asked me, the magazine’s lawyer, whether there was any legal danger in doing so. I advised that we publish. Almost immediately after the article, Thorpe’s Trial: How the Jury Saw It, appeared, Margaret Thatcher’s attorney general prosecuted us for contempt of court. This was an ill-defined judge-made law by which the legal establishment could punish actions that tended to pervert justice – although the only action that had perverted justice in the Thorpe trial was that of the Sunday Telegraph.

The case was heard by Lord Widgery, the fearsome chief justice. He was, by 1980, suffering from dementia – another fact that the legal establishment covered up. He read his law books upside down, asking me several hours into the case, “Who is Mr Bruce Page?” Fortunately the colleague who covered for him was a good, principled common lawyer, who accepted my argument. The Statesman was acquitted.

At the time, the decision appeared a great victory for freedom of speech. But the government rushed through a new criminal law – section 8 of the 1981 Contempt of Court Act, still on the statute book – which threatens jurors and journalists with up to two years’ imprisonment should they disclose “any particulars of statements made, opinions expressed, arguments advanced or votes cast” in the course of jury deliberations. It was feared that the jury system would not survive if subject to rational investigation. Section 8 has in the years since prevented exposure of miscarriages of justice, and stopped research into (for example) whether complex fraud cases are suitable for jury trial.

Americans cannot understand the British obsession with secrecy – for them, jury service is a public duty, of obvious public interest if anything goes wrong in the jury room, or even if it doesn’t. Jurors can write books about their experience. In Britain, as if for fear of letting daylight into a system that may or may not be working, we jail those who blow the whistle.

If you do jury service and your fellow jurors decide the case on the toss of a coin, or by reference to racist or homophobic comments about the defendant, you will go to prison if you tell the media. It might be acceptable to ban disclosure of the names of fellow jurors, or to prevent the identification of the trial itself unless there were a public interest in doing so (as with the Thorpe case). But a blanket ban on jurors disclosing, or journalists publishing, details of justice miscarriages shows the length to which the British will go to stop the exposure of scandals about our beloved institutions.

• Geoffrey Robertson is a human rights barrister. His autobiography, Rather His Own Man, is published by Biteback this month