The killing of a Bulgarian dissident, Georgi Markov, on Waterloo Bridge in September 1978 is a case to clinch the truism that real life is better scripted than fiction. As Markov was crossing the Thames, heading back to BBC headquarters, he bumped into a man with an umbrella and felt a sharp pain in his leg. The man apologised, got in a taxi and disappeared.

Four days later, Markov was dead. Doctors discovered a pinprick irritation on his right thigh. They removed the flesh and found a 1.7mm pellet containing 0.45 mg of ricin, a poison that had almost certainly come from Russia.

From the start the story seemed staged and stylised: a refugee intellectual from behind the iron curtain, now working for the BBC, had been killed in the British capital using an object forever associated with London: a brolly. It later emerged that the assassin had been given the codename Agent Piccadilly. Markov had been attacked on 7 September, the birthday of Todor Zhivkov, Bulgaria’s long-term, de facto dictator.

Ulrik Skotte has been on the story since the early 1990s, when he was a rookie journalist working at DR, the Danish broadcasting corporation. He had contacted a perky, anarchist film-maker, Franco Invernizzi, who lived in Copenhagen and who boasted that he knew the real murderer: another Italian who lived in the Danish capital.

The book recreates the early conversations between Skotte and Invernizzi to perfection. Given the decades that have passed, you doubt they are word-for-word authentic, but they convey very convincingly the character of both men. Skotte is eager but frustrated by the older man’s procrastination and his inability to deliver on the promise of an interview with the assassin. Invernizzi is covetous of his story: he’s big-hearted, but paranoid and as the years pass he gets sucked into the quicksand of intelligence conspiracy theories.

Skotte later became a documentary maker, and he paces and structures this book masterfully, jumping back and forwards to fill out the canvas like an impatient painter. Markov emerges as a womaniser with a betting habit, but also as someone who had once been on close personal terms with Zhivkov. The two men used to walk and dine together, which is perhaps why Markov’s subsequent takedowns of Zhivkov from London hurt him so much. There are deft pages on Russia’s mokroye delo, or “wet jobs” – poisonings that evolved from the work of Grigory Mairanovsky, a sadist who had pioneered ways to administer curare, thallium, dimethyl sulfate and ricin to enemies of the Soviet Union.

But the central character of the book is the assumed assassin. Francesco Gullino was born – it does get a bit James Bond at times – in Bra, Piedmont, in 1945. He was brought up by his aunt, who either ran a bar or a brothel, depending on your definition. Sent to a religious secondary school run by monks, he became, in his teens, a wood carver, equestrian and chameleon.

In his 20s he started selling secondhand paintings along the Tuscan coast and the Côte d’Azur. He worked for a Turin-based import-export firm, Mondial, that was thought to have links to far-right organisations. Gullino was a confirmed fascist (he carried copies of Mein Kampf) and had soft-porn proclivities: he liked to pay sexworkers to pose for him and was responsible for publishing the erotic novel, Emmanuelle, in Italian.

Gullino had an uncanny ability to calm horses, but was also able to slip into different characters and countries. An Italian fascist working for the Soviet side in the cold war, he veered inexplicably from squalor to international deal-making. He had stacks of empty frames and ran some kind of stolen/counterfeit art scam that had connections to Palermo and, believed Invernizzi, organised crime.

At some point in the mid-70s, Gullino was recruited by Bulgarian secret services in order to duck a smuggling accusation (he had been caught with narcotics, it’s thought, returning to Italy from Turkey). All these details only emerged 20 years later when Soviet archives were opened and various Bulgarian spooks began to testify.

Skotte and his team – he’s generous about the collegiate nature of research – did the hard miles. They interviewed Zhivkov outside Sofia with courtesy but bluntness. He denied any involvement, naturally, but Skotte’s character sketch is in some ways better than a confession. The author also taped a three-hour interview with Gullino a month before he died – as close to closure as the book gets. Gullino walked a fine line between boasting and denying, toying graciously, sometimes childishly, with his interviewer.

There’s no clinching piece of evidence, of course, and Skotte realised, like Invernizzi before him, that Gullino would never be tried. It’s true that he had been in London in September 1978, and he had been interviewed by British and Danish investigators in the 1990s. But in later years he became more brazen, turning himself into a caricature, posing with an umbrella for photoshoots and milking his notoriety.

Two other deaths lend weight to the evidence against him, though. In 1990, a young sex worker was murdered in Copenhagen. Among the victim’s possessions was a photograph of herself riding a horse (“with Gullino” was written on the back). Gullino provided a false alibi, claiming to have been at a party whose host Skotte tracks down: they deny Gullino was there that night.

Invernizzi himself died in suspicious circumstances. He had appeared in a documentary badly anonymised and had named Gullino as the Markov murderer (until then the Danish media had only used the initials FG). That night, the two men had dinner and Gullino was, according to Invernizzi’s widow, “furious. He felt that Franco [Invernizzi] had betrayed him by going on camera. So they went out to Nordsjælland and ate at some restaurant there. The next day, Franco wakes up feeling terrible. After that, he goes to the hospital. And a day or two later, he’s dead.”

Most of us on the Italian true-crime beat have always assumed that the most compelling Italian crime story linked to London was the murder of Roberto Calvi, found under Blackfriars’ Bridge in 1982. But Skotte tells the Markov story so crisply that this other bridge killing, in 1978, seems even more poignant: somehow more international, evocative and satisfyingly odd.

Tobias Jones lives in Parma. The TV adaptation of his Blood on the Altar has just been broadcast by Rai 1 in Italy

• The Umbrella Murder: The Hunt for the Cold War’s Most Notorious Killer by Ulrik Skotte is published by Ebury (£25). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply