To outline the facts: Russian exile and former secret service agent Alexander Litvinenko died in London on November 23, 2006, after being poisoned with a huge dose of polonium, the victim of - depending on a variety of conspiracy theories - the Kremlin, Russia's Federal Security Service (FSB), Vladimir Putin, former business associates or the shadowy forces behind any or all of the above.

Veteran journalist Murray Sayle once said that there were two sorts of really good news story: A) arrow points to defective part; B) we name the guilty men; and that the former has always been easier than the latter. Martin Sixsmith's The Litvinenko File covers A more than adequately, but falls short on B.

The book is disappointing because it hovers between the realms of supermarket blockbuster and hard-hitting investigation.

Sixsmith, a former BBC journalist, doesn't seem entirely comfortable with writing and pads out his material with glossy description. And, while supposedly nearing the end of the trail in Moscow, has lengthy chats with his taxi driver.

Confusingly, there are no footnotes or index, no chronology or even a list of the cast of colourful characters, leaving readers to try to recall whether Yuri Felshtinksy, Yevgeny Khokholkov, Akhmed Zakayev and Anna Politkovskaya were in some way implicated or just played supporting roles.

These flaws aside, the book is an intriguing portrait of the ebb and flow of Russian power in recent years, aptly summarised by the word bespredel, meaning lawlessness within the state.

One of the principal characters, Boris Berezovsky, rose from maths professor to become one of the country's richest men and a close ally of former president Boris Yeltsin. His star waned under Putin, so much so that he fled to Britain and later arranged for Litvinenko - one of his proteges - to follow him to London after he had similarly fallen foul of authority.

Other such oligarchs remain in Russia and from Sixsmith's narrative it is clear that extortion and even murder are viewed as almost routine business methods in the more unscrupulous tiers of the country's commercial world.

Such is the maelstrom that gradually enveloped Litvinenko. Without giving too much away - and the book is worth reading, despite its textual annoyances - he became disillusioned with the cause he was supposedly fighting for while on operations in Chechnya. His appearance in November 1998 on the platform of a press conference to denounce the workings of the FSB (which had replaced the KGB) - an act that exemplified his stubborn yet naive character - marked him out as a traitor from the viewpoint of several parties.

Even after he had escaped to London and was living in a house in Muswell Hill, which Berezovsky had paid for, threatening phone calls from Moscow would remind him of the fate of Leon Trotsky, who was hacked to death with an ice pick on Stalin's orders in 1940.

The British press leapt on Litvinenko's assassination, revelling in a tale that combined James Bond with nuclear terrorism on the streets of London.

Sixsmith's approach is more level-headed and his contacts - one takes him to a poison laboratory in Moscow - are formidable.

He does a good job of portraying Litvinenko the man, discounting his more fantastic accusations like 'Putin the pederast' and concentrates on his increasing isolation while expatriated in London. Although Sixsmith names a number of potentially guilty men - and Scotland Yard has fingered a definite suspect - the book concludes, with a touch of inevitability, in Highgate Cemetery, where Litvinenko lies buried a short distance from the bust of Karl Marx.