From the ousted Thai PM to fracking company Cuadrilla and Asma al-Assad, the PR firm Bell Pottinger represents some of the world's most notorious clients. Tim Bell, who masterminded Margaret Thatcher's election campaigns, explains why he thinks his business is a force for good

At first sight, the Curzon Street offices of Bell Pottinger Private are everything a conspiracy theorist could hope for. Next door, guarded by uniformed police officers with machine guns, squats the immense embassy of Saudi Arabia, a country with which the controversial PR firm's even more controversial chairman, Tim Bell, has had decades of opaque, often defence-related dealings. Beyond the embassy is the rest of Mayfair, where businesses are discreet, ethics are flexible, and rich people with reputations to upgrade are in increasingly plentiful supply.

In the lobby of Bell Pottinger, there are cream leather benches for visitors. The day's newspapers hang from clips on the wall. The implication is that Bell, who has worked in PR or advertising since 1959, can get you or your company or government into them – or quietly arrange the opposite. "We keep a lot of people out of the media," says James Henderson, the firm's chief executive.

From the lobby, a tiny lift whisks you up to Bell's top-floor office. It is a long corner room, with a row of windows looking out over Mayfair's wine-red and bone-white rooftops. In the centre of the room is a large desk, almost in the shape of a quotation mark. Along its outer curve are a few chairs for clients. On the inner, there is an oldish computer, two telephones, two used coffee cups, a half-full ashtray and Bell himself.

Lord Bell of Belgravia, who was knighted by his longstanding client and political soulmate Margaret Thatcher in 1990, is 72. But he is still working full-time, and still looks like an old-fashioned PR man from central casting: slicked-back hair, assertive tie, tailored shirt, quick smile. He talks in a half-gravelly, half-chocolatey voice, and instantly drops your first name into his quick sentences. "He is an icon in the business," says Mark Borkowski, another British PR veteran and a historian of the industry. "A lot of people have tried to write his obituary, but they underestimate his steely determination. He has a very powerful network, contacts all over the world. Bell Pottinger are a formidable multinational. For many, many years they have operated in the shadows, as agents of influence."

'Labour Isn't Working' campaign poster, which helped discredit the Callaghan government in the late 70s.

Bell grants interviews rarely, and then usually to rightwing newspapers, his preferred journalistic conduits. But sometimes he wants to reach a wider audience. Today, sitting back from his desk in a shaft of cigarette-fogged sunlight, he is in expansive, self-deprecating mode.

"The Thatcher legacy – of course I live off it to some extent," he says. "What I did for her has been grossly exaggerated by some, and grossly underestimated by others." According to Mark Hollingsworth's biography of Bell, The Ultimate Spin Doctor, he advised her on everything from how to relax on television – "to melt her almost frozen expression, Bell would sit behind the camera and pull faces" – to how to attack Labour, including the famous "Labour Isn't Working" poster campaign, featuring an endless queue of the unemployed (actually Young Conservatives borrowed for the shoot), that helped discredit the Callaghan government in the late 70s – shortly before the Thatcher government sent unemployment much higher. "My profound belief," Bell continues, "is that a small number of words, a strong visual image, can change the way people think." At Bell Pottinger, which he co-founded in 1998, "We tell stories – I don't mean lies. We work for people who want to tell their side of the story."

The government of Sri Lanka; FW de Klerk, when he ran against Nelson Mandela for president of South Africa; Thaksin Shinawatra, the ousted Thai premier, whom protesters claim still controls the country; Asma al-Assad, the wife of the president of Syria; Alexander Lukashenko, the dictator of Belarus; Rebekah Brooks after the phone-hacking scandal broke; the repressive governments of Bahrain and Egypt; the American occupying administration in Iraq; the polluting oil company Trafigura; the fracking company Cuadrilla; the athlete Oscar Pistorius after he was charged with murder; the Pinochet Foundation during its campaign against the former Chilean dictator's British detention; the much-criticised arms conglomerate BAE Systems – Bell or Bell Pottinger has represented all of them. "They get involved in lots of 'special situations', in reputational and crisis PR," says Alec Mattinson, the deputy editor of PR Week. "The reputation they have is as an agency that goes where other agencies would fear to tread."

A Buddhist monk puts on a gas mask during clashes in Bangkok. Photograph: Dylan Martinez/Reuters

This notoriety has had consequences. In 2011, the Bureau of Investigative Journalism and the Independent published a Bell Pottinger exposé, including covertly filmed company executives (though not Bell or Henderson) bragging about their influence over the Sri Lankan government and the British Conservative party, and about the firm's expertise in "all sorts of dark arts". This August, anti-fracking protesters superglued themselves to the doors of Bell Pottinger's other London office, on a busy street in Holborn, during the morning rush hour. Last month, the BBC3 satirical show The Revolution Will Be Televised broadcast its attempt to gatecrash this year's Bell Pottinger summer party – a Whitehall gathering of dark-suited men with worldly auras – by a comedian dressed as the Devil and then as Hitler.

Bell and the company have a range of responses to such frontal attacks. One is to claim victimhood: "The left is extremely strong, and very active, and very keen on demonstrating its point of view by shouting at people," he says, with a world-weary shrug. "It's become very fashionable to shoot the messenger."

Another approach is to affect casualness. When I ask if Bell Pottinger is still working for the Sri Lankan government – citing commercial confidentiality or official secrecy, the firm does not publish a full list of clients – Bell says airily: "We stopped in … 2009? Or 2010? I might have got the dates wrong."

Then he switches seamlessly to a sterner, man-of-the-world tone: "It's a fashionable thing to criticise the way the Sri Lankan government has behaved. David Cameron had one meeting in the north of the country with 200 people who have lost relatives. You have to remember there was a 30-year civil war. The Tamil Tigers weren't exactly gentle, nice people. And for Britain to ponce around the world talking about human rights after what we did in Afghanistan … It's what Winston Churchill called 'our usual export': hypocrisy."

Finally, and most ambitiously, Bell claims that his company is in fact a force for good. "We are actually decent people," he says. "We do know right from wrong. The reason we worked for Lukashenko in Belarus was because he told me: 'We want to go along the path to democracy.' We actually got six political prisoners released."

Alexander Lukashenko, president of Belarus. Photograph: Pool/Reuters/Corbis

The shameless incompatability of all these arguments is typical Bell: always slippery, always trying a new line on you, and, deep down, probably trying to persuade himself. "He's mercurial, quite emotional," says Hollingsworth. "He can suddenly turn."

Yet the question that hovers over Bell Pottinger is whether this theatrical, highly personalised approach exactly meets the needs of a modern PR multinational. After 20 highly quotable minutes in Bell's office, we are joined by Henderson. The chief executive, who only came to the company three years ago, is a quarter of a century younger than Bell, and has a much more sober background in financial and corporate PR. Unlike Bell, he keeps his suit jacket on.

At first, while Bell talks and talks, Henderson stays quiet, rolling a pen between his fingers. Then, just as Bell is concluding a long, involved anecdote about why he turned down a chance to work for the authoritarian president of Zimbabwe, Robert Mugabe, Henderson interjects: "We have a very, very non-controversial client base now," he says. "Our client base doesn't reflect what you read about in the press."

The Bell Pottinger website mentions work for dozens of less-than-sinister entities such as Kurt Geiger, Daylesford Organic and the London Chamber Orchestra. Yet many of the commercial sectors the company lists as focuses for its PR activities – "oil and gas", "mining", "financial institutions", "Russia" – suggest a readiness to work at the more rugged end of international capitalism. Your attitude to the company doing such work, says Bell, flashing a wide smile, "does depend on your definition of controversial".

Henderson argues that Bell Pottinger is disproportionately targeted by critics of London's hard-nosed PR world. "These 'controversial' clients – we do pitch for them against other PR companies," he points out.

PR Week's Mattinson agrees: "The idea that they're the only company prepared to work for controversial clients is a [mistake]." In the magazine's league table of PR firms operating in Britain, Bell Pottinger is at number five; Hill & Knowlton, a bigger American rival, controversial since the 30s, which has also promoted repressive governments and fracking, is just behind at number seven. Portland PR, a newer British firm set up by Tony Blair's former aide Tim Allan, has already achieved some notoriety for its work for authoritarian Kazakhstan and Russia.

One reason, perhaps, why Bell Pottinger is singled out for special criticism is its lingering air of rightwing tribalism. When I ask Henderson why they haven't ever worked for controversial leftwing governments, such as the many currently in South America, he looks at me slightly uncomprehendingly, then says: "We've never been approached by a leftwing government, that I'm aware of." Bell adds: "You don't want an adviser that doesn't agree with you."

Tim Bell and James Henderson of Bell Pottinger. Photograph: Sarah Lee for the Guardian

Bell grew up in the aspiring, often politically conservative, outer suburbs of north London. His background was entrepreneurial and middle class: his Irish father was a gifted salesman and his Australian mother came from a shop-owning family, like Thatcher. As a young man during the 60s, Bell was less interested in politics than in his advertising career, and the flash clothes and cars it paid for, but he found time to canvass for several north London Tory MPs, including Thatcher. When she hired Bell and his then employers Saatchi & Saatchi to work for her party in 1978, he and she immediately established a rapport. He became one of the closest in her inner circle of clever, often slightly roguish informal advisers, partly because they had a similar worldview: us-against-them, fiercely anti-communist, unquestioningly pro-market. "If anyone inspired me, it's Ayn Rand," says Bell, namechecking the famously confrontational American rightwing author. Long after the fall of the Berlin Wall, Bell still privately referred to journalists he disliked as "commies" and "pinkos".

Henderson is less of a crusader. Privately educated, unlike Bell, more smartly spoken, more quietly dressed, he could be a Mayfair hedge-fund trader. "We are all supportive of Tim and his politics," he says with a slightly fixed smile. Bell Pottinger's partisan and more straightforwardly corporate impulses, he goes on, "can work well side by side". In a sense, this is right. Bell's work for the Conservatives in the 70s and 80s helped create a world – of London as a hub for the international rich, of the privatised utilities, of a post-communist, status-hungry eastern Europe – that has produced clients for Bell Pottinger, and Bell's previous PR enterprises, ever since.

Much of what Bell Pottinger does for these clients is commercially confidential, but the company also needs to promote itself, and its website offers surprisingly detailed case studies. "Katrin Radmacher is a German heiress," begins one. "She asked Bell Pottinger to help uphold her reputation and manage the media during a legal marathon with her former husband … After each ruling, Bell Pottinger issued a highly quotable statement on her behalf to the world's media … When misconceptions arose, Bell Pottinger quietly briefed the press. A Tatler interview, establishing her as a privileged, yes, but unassuming mother of two who'd tried to save her marriage, was syndicated in the Sunday Times. Evening Standard and FT interviews, and a Times leader, were sympathetic …"

A General Pinochet supporter. Photograph: Santiago Llanquin/AP

Bell Pottinger likes to portray itself as a dexterous media manipulator, yet sometimes the process is more fraught. In 1998, the campaign it helped orchestrate for the release of General Pinochet had a clever slogan – "reconciliation not retribution" – but also internal tensions. In 2000 Charles Alexander, a pro-Pinochet figure in the City of London, told me: "I said on the first day after he [Pinochet] was arrested [in Britain], 'He's got to get very ill, very quickly.' But Tim Bell disagreed with me. He said: 'Pinochet got rid of the commies, and that's our argument.'" In the end, the campaigners settled for an awkward mixture of both approaches, apolitically playing up Pinochet's frail health while also producing and distributing crude rightwing propaganda pamphlets and articles about the elected leftwing leader he had overthrown, Salvador Allende. The general eventually flew back to Chile, but with his international reputation even lower.

Nowadays, Henderson's strategy for Bell Pottinger sometimes seems more safety-first. Last year, he, Bell, and the firm's co-founder, Piers Pottinger, a City of London PR veteran, organised a management buyout of the company, extricating it from the bigger communications conglomerate Chime. Since then, Henderson has emphasised "rebuilding the Bell Pottinger brand", getting the firm "up the league tables", and winning a greater "number of mandates [contracts]". Much of this strategy is being enacted not in Mayfair, but in the Holborn office, which houses Henderson and nine-tenths of the firm's 250 staff. The Bell Pottinger premises there have a security guard in the lobby – perhaps on the lookout for more protesters with superglue – but otherwise seem a little more prosaic: a single floor of a heavy postwar office block, above a Sports Direct and an M&S.

In some ways, the PR business is getting harder and less glamorous. The internet and particularly social media are making Bell Pottinger's traditional publicity channel – the newspapers hung up in its Mayfair office, long susceptible to Bell's smooth phonecalls – less central to the making and unmaking of reputations. And the sour mood in many countries towards politicians, big business and the wealthy is making the public less ready to accept that such interest groups are not, as Bell Pottinger would put it, really such bad guys after all. "The most difficult thing now in our business is having an impact," Bell acknowledges. "You have to operate in more and more areas of distribution."

Bell is a professional optimist, but occasionally seems gloomy about the modern world. "I hate the internet," he told the authors Charles Vallance and David Hopper in a recent book on British entrepreneurs, The Branded Gentry. David Cameron does not enthuse him: "I don't know him, and I don't understand him. He's obviously stuck in the Lynton Crosby strategy: be vile, and that'll do." Of course, this may be a piece of pragmatic positioning, as Cameron's chances of re-election begin to diminish. When I ask about Ed Miliband, Bell says brightly: "He's obviously very clever, and has done some smart things." But he goes on to praise George Osborne for his "clear" free-market thinking. For all Bell's gut political feelings, Hollingsworth's biography shows he has long been willing to play situations both ways. In 1985 he told Media Week magazine: "I want the BBC to fail"; a month later, he took on the contract to do their advertising.

At Bell Pottinger, clients with opposing interests are assigned separate PR teams, and the situation is kept manageable and ethical – at least by Bell Pottinger's standards – by internal "Chinese walls". Clients are always informed when the firm is representing a rival, says Henderson, but few walk away. Bell Pottinger's ease with conflicts of interest, like its readiness to represent dubious clients, is easy to find chilling; but to detail and condemn it can act as a form of free advertising: there will always be people in the world who want an unsqueamish PR firm. Likewise any article that portrays Bell Pottinger as having tentacles everywhere: "I'm delighted to have people think we have a finger in every pie," says Henderson, finally smiling. "Because that's our objective!"

In truth, both Bell Pottinger and their critics often overstate the firm's power. Henderson claims its work for Cuadrilla has "completely changed the whole debate" about fracking. It doesn't quite feel like that. Nor does it feel like world opinion has softened much over the decades towards Bell Pottinger's contentious government clients. In April, PR Week reported that the firm had "annual revenues [in] the mid/high £30m mark" – high for a company with 250 employees, but modest compared with other rightwing media players such as Rupert Murdoch.

Yet for Bell at least, precisely measuring the effect of Bell Pottinger's work – even if that were possible in the infinitely subjective world of PR – is probably not the point. "I don't think he's ever been interested in being [part of] a big PR conglomerate," says biographer Hollingsworth. "His life is about the buzz."

Outside his top-floor office, the sun has gone in and a grey winter afternoon is settling over Mayfair. Our interview is past the hour and a half mark, yet Bell is still spinning tirelessly. "My interest is in high-profile news," he says, unleashing another smile. "It's nice to be part of what's going on." On the pavement below, the police officers outside the Saudi embassy pace up and down with their machine guns.