Nearly a generation has passed since a London postman on his way to work discovered the body of a well-heeled, nicely-turned out Italian businessman hanging by the neck from scaffolding under Blackfriars Bridge.

The businessman was a banker called Roberto Calvi. His body was discovered at 7.30am on 18 June 1982. Ex-banker would be more accurate, because the day before he died Calvi had been relieved of his duties at Banco Ambrosiano, of which he had been chairman, and his secretary had jumped to her death. The bank was about to collapse with £800m in debts.

Some of Ambrosiano's debts were to people one would not choose to be indebted to, like the Sicilian Mafia. Calvi, in London under a false name and on a bogus passport, was perhaps looking for a way out and a new life, a life where he would be liberated from the consequences of his appalling mistakes. If that was the case, he didn't find those things. Or perhaps he just had no idea where else to go, and was terrified of what might happen next: it was said that he spent much time on the telephone in his residential hotel and very rarely went out. He had shaved off his moustache in an attempt to disguise himself. In the end, all Calvi found in London was a rope around his neck and a dismal sort of immortality.

Italy has been the land of mysteries since the time of the Medicis, and in the death of Roberto Calvi it exported a prize specimen to England. The jury at his inquest decided he had committed suicide. A second inquest, forced by his family, returned an open verdict, unable to decide between suicide and murder. In Italy most informed observers were convinced that Calvi's death was murder - though by whom was another question altogether. And today, more than 23 years after that day in June 1982, five people are finally to go on trial in Rome accused of killing him.

The ingredients of the trial are explosive. During the heady years of Italy's dynamic growth in the Sixties and Seventies, Roberto Calvi made a brilliant career at the head of Banco Ambrosiano. He was a brilliant financier, and though shy and socially gauche he combined a lightning accountant's brain with recklessness in a very Italian fashion. He obtained the best, the most desirable and discreet clients in the country, and also the worst, and he hung on to all of them by giving them exactly what they wanted: fat rates of return, immense discretion, an insouciant willingness to do what others would not dare. He was a devout Catholic, and gained a special place of trust with the Institute for the Works of Religion, the Vatican's bank. But he was also the banker to the Sicilian Mafia. He was also, it is charged, intimately involved in P2, the secret Masonic lodge, "Propaganda Due" (2), which brought together most of Italy's high-flyers in politics, business, the civil service and intelligence (Silvio Berlusconi was a member for a time), to the mutual interest of all.

But a series of errors of judgement and strokes of bad luck sent Mr Calvi's handiwork crashing down. Huge sums of money disappeared into illegal offshore accounts, particularly in Panama. Highly placed, highly compromised clients in P2 fled the country. Ambrosiano's illegal dealings, its massive debts and offshore accounts were exposed. The Bank of Italy, 25 years ago a tough and principled regulator, sent in its investigators.

Banco Ambrosiano was threatened with going bust with debts amounting to hundreds of millions of pounds, and suddenly all Calvi's high-flying friends became serious liabilities. He was put under investigation for illegal foreign currency dealings; while in jail under investigation, he attempted suicide, taking an overdose and slashing his wrists. He was convicted and sentenced to four years in jail.

Although he was now out of jail pending appeal, Calvi dreaded losing the appeal and being sent back to serve his sentence. As the date for the appeal drew near, he must have asked himself, what options did he have?

On 11 June 1982 he took the most reckless way out. He secretly fled Italy for London with a black briefcase full of his most incriminating documents.

He was accompanied by a cigarette smuggler from Trieste by the name of Silvano Vittor, acting it appears as his minder, and a Sardinian businessman called Flavio Carboni: two of the accused who go on trial in Rome today.

Nobody has succeeded in explaining why Calvi needed to be in London, but it was certainly not a casual trip. The arrangements read as if they had been lifted from a Tintin adventure: a private flight from Rome to Venice in the middle of the night, Venice to Trieste by car, a switch of identities for the trip by speedboat from Trieste to an abandoned pier in Yugoslavia, a sojourn in a chalet in Austria, and finally his arrival at an airport in England, masquerading as a Fiat executive. All for what?

In London Calvi and Vittor checked into a second-rate residential hotel at 881 Chelsea Cloisters, paying the un-bankerly rate of £40 per night. And there he stayed until his death. In London, Roberto Calvi was the invisible man. Nobody the police interviewed remembers seeing him, nobody admits to meeting him, no records remain of what he said and did there. The police have admitted they had no idea how he got from the Chelsea Cloisters to Blackfriars Bridge. All they had to go on was the mysterious spectacle with which they were presented when they cut the man down.

The Italian was suavely dressed in his own topcoat. His expensive Patek Philippe watch was still on his wrist, loafers by the same firm were on his feet, lapped by the river water. Around his neck was an orange noose. In his wallet was about £10,000 in sterling, Swiss francs and Italian lira. Stuffed into the pockets and down his flies were bricks and stones that the police believe came from a nearby building site.

The presence of the money and the watch appeared to rule out a mercenary murder. At the same time, the coroner found no marks on Calvi's body indicating he had been the object of violence before his death, no syringe marks to suggest he had been drugged, no drugs in his system besides the residue of the single sleeping pill he had taken the night before.

So he had not been mugged or beaten or sedated before being strung up; he had eminently good reason to kill himself, given the prison sentence, the collapse of his life's work, the loss of his job, his disgrace - and he had tried to do it once before. So when the jury filed a verdict of suicide, it must have made perfect sense.

It did not, however, make sense to Calvi's closest kin, his widow and his son and daughter, who believed from the outset that Roberto Calvi had been killed. They challenged the original inquest - but at a second one, held in London in 1983, the conclusion was that it was impossible to say whether Calvi had killed himself or been killed by others.

Yet Carlo Calvi, the banker's only son, who was studying for a doctorate at Washington's Georgetown University when his father died, refused to give in. He - like informed opinion in Italy - was sure that his father had been killed. The fact that this was as close to the perfect murder as has ever been achieved did not change his view. So in 1989 he hired a firm of private detectives to take the forensic investigation further than had the City of London Police.

Kroll Associates located the scaffolding poles from which Calvi had been suspended, re-assembled them exactly as they had been under Blackfriars Bridge, and then had a stand-in for Calvi, of the same height and weight, take the route that the Italian would have to have taken if he really had ended his own life at the end of the orange rope.

The detectives were not interested in the factors that had already convinced Carlo Calvi that his father could not possibly have killed himself this way. Roberto Calvi was 62 when he died, overweight, and a chronic sufferer from vertigo. In the pitch darkness he would have had to spot the scaffolding under the bridge, practically submerged in the high tide, stuff his pockets and trouser flies with bricks, clamber over a stone parapet and down a 12ft-long vertical ladder, then edge his way eight feet along the scaffolding. He would then have had to gingerly lower himself to another scaffolding pole before putting his neck in the noose and throwing himself off, because both inquests noted that there was minimal damage to Calvi's neck, indicating that he had not dropped a long way. A suicidal acrobat might have pulled all this off, supposing he was sufficiently determined - but a fat, vertigo-prone banker?

But Kroll Associates were not interested in what was probable, only in what was unavoidable. They had their Calvi stand-in wear the same kind of loafers the banker was wearing when he died, then manoeuvre his way on to the scaffolding by the various possible routes: after which the shoes were soaked in water for the same length of time as Calvi's.

Each time the test was tried, microscopic examination of the shoes by a forensic chemist picked up traces of the yellow paint with which the scaffolding poles were stained. And as the shoes Calvi was wearing when he died betrayed no such traces, Kroll concluded: "Someone else had to have tied him to the scaffolding and killed him."

As a result of Carlo Calvi's long campaign to clear his father from the dishonour of suicide, in September 2003 City of London Police reopened the case as a murder inquiry, and with the aid of new forensic techniques unearthed the evidence - from the scaffolding poles and from Calvi's exhumed remains - that ultimately led to the opening of the trial in Rome today. As the inquiry reopened, Detective Superintendent Trevor Smith said: "We have been applying 21st-century forensics and investigative techniques to a 21-year-old crime." Going on trial today in a special court in Rebibia prison, on the outskirts of Rome, are Carboni and Vittor, the two men who accompanied Calvi to London and who vanished from Britain on the day his body was discovered. On trial with them are two Italian gangsters who were also in London at the time, Pippo Callo, a convicted mafioso who is currently serving a sentence, and Ernesto Diotallevi, a former boss with Rome's notorious Magliana Gang.

Carboni's former girlfriend, an Austrian called Manuela Kleinszig, is also in the dock.

Yesterday one of the prosecutors in the case, Luca Tescaroli, told The Independent: "After 23 years it is a great success that this trial is taking place. We will keep on with our investigations: this is just the starting point, there will be further developments, other names will be added to the list of the accused. A lot of unsettling history in our country is going to be exposed."

It is undoubtedly a major achievement that Calvi's murder has finally been exposed for what it is. Whether the trial will succeed in unravelling the full mystery of Calvi and his bank is, however, open to doubt. As in so many other great Italian mysteries of the past 30 years, forensic science can only achieve so much. It may pin the blame on a few nasty thugs, who may even do time for it. How far it will succeed in pinpointing the powerful men who tugged their strings is quite another matter.

The story so far:

1971

Roberto Calvi becomes chairman of Banco Ambrosiano

May 1981

Calvi is convicted of corruption in one of Italy's largest banking scandals but is given bail.

11 June 1982

Skips bail and travels to London on a false passport

18 June 1982

His body is found hanging under Blackfriars Bridge.

July 1982

Inquest records a verdict of suicide.

July 1983

Second inquest records an open verdict. Throughout the Eighties, a number of books are published about Calvi's death, amid rumours of Mafia involvement.

November 1992

Charles Raw, chief investigative financial reporter for the Sunday Times, claims to link Calvi's money-laundering with a right-wing Masonic organisation - P2 - and the Vatican.

1998

At his family's request, Calvi's body is exhumed.

April 2002

Several Italian newspapers report leaked findings by Professor Bernd Brinkmann of Münster University that suggest Calvi was murdered.

October 2002

Forensic experts in Rome conclude Calvi's death to be murder. Neck injuries are said to be inconsistent with hanging.

July 2003

Italian prosecutors release the names of four suspects: Flavio Carboni, his Austrian ex-girlfriend Manuela Kleinzig, Pippo Calo and Ernesto Diotallevi. All are accused of having Mafia connections.

September 2003

City of London police agree with the Italian findings that Calvi was murdered.

December 2003

A 42-year-old woman is arrested by British police investigating her testimony at the original inquest. She is later freed on bail.

March 2004

The four suspects appear at a pre-trial hearing.

May 2004

Paul Matthews, the City of London coroner re-examining the case, has his bags and laptop stolen in Rome. Italian authorities suspect Mafia involvement.

April 2005

Suspects, including Silvano Vittor, are charged with murder.

6 October 2005

The trial begins

Jerome Taylor