When Paul Mason was covertly recorded by the Sun newspaper divulging his private view that Jeremy Corbyn does not appeal to the working classes, there wasn’t much surprise in the Labour leader’s office. The relationship between Corbyn and his celebrity guru has always been complex. Kremlinologists point to a meeting of Corbyn’s closest comrades earlier this year at Esher Place, a £6 million, Grade II country house in Surrey owned by Unite.

The guest list was a who’s who of the hard left: John McDonnell, Diane Abbott, Len McCluskey, Labour strategy chief Seumas Milne and Momentum boss Jon Lansman were all in attendance. Corbyn had also invited Mason to join this star chamber. Those present saw it as confirmation that the former BBC and Channel 4 journalist is acting as an unofficial adviser to the Labour leadership.

‘Jeremy listens to him,’ explains one Corbynista. ‘The only people he ever listens to are his sons, Seumas and the old guys he’s known for years. For him to listen to Paul is really something.’

Yet Mason’s counsel received mixed reviews. Corbyn and McDonnell were impressed by his detailed suggestions on economic policy and his vivid analysis of how left-wing movements can sweep to power across Europe. Less popular are his views on Trident (pro-renewal), Russia (anti-Kremlin), Nato (pro) and mass immigration (critical). Personalities have clashed; colleagues remark that you could not find two more different people than Milne, an exceedingly polite Old Wykehamist, and the plain-spoken Mason.

In September, the leader’s office was deeply-unhappy about a very public row Mason had with the Labour MP Anna Turley, in which he called for her to be deselected. It’s no secret that the Corbynites are limbering up to unseat Labour moderates, but Mason’s outburst was seen as overly aggressive, macho and unhelpful. ‘He’s a good economist, not so much of a political strategist,’ says a source. The Sun tape revealed Mason’s choice to succeed Corbyn is Clive Lewis, the former shadow defence secretary who was demoted after a row about Trident at Labour conference. Mason and Lewis are close allies, so much so that Labour sources suspect his hand in the conference bust-up. They claim Mason lobbied Lewis to back the nuclear deterrent, forcing Milne to remove the offending line from the autocue at the last minute. Mason is said to have felt ‘aggrieved’ by the incident.

A friend admits he is ‘detached’ from some of those closest to Corbyn: ‘He’s shocked about how incompetent some of them are.’ So how on earth did the former-economics editor of Newsnight, that bastion of BBC impartiality, end up as a major player on Labour’s hard-left? Born in Lancashire to a lorry driver and a half-Lithuanian primary school teacher, Mason’s upbringing left him with a strong regard for the internationalist, European left, as well as the working-class values of 1960s northern England.

His economics were self-taught: he originally trained as a musician. A huge Wagnerian, Mason dreamt of being a classical composer and worked as a music teacher before becoming a journalist. But he was always political — in 1987 he was filmed by ITN handing out Workers’ Power, a Trotskyist newspaper, and talking about ‘building revolutionary politics within the Labour party’. It wasn’t until his thirties that he became a journalist, working at Computer Weekly before he was talent-spotted by the BBC.

Former Newsnight colleagues praise Mason’s professionalism and claim his coverage seldom betrayed his politics. ‘He was putting grown-up economics on TV at a time when the boss class were completely against it,’ comments a friend. Mason is now a dynamic, engaging broadcaster, but he was ‘particularly bad on camera’ early on. He overcame his nerves by doing voice exercises before going on air, much like an actor warming up backstage — something he still does to this day — and these are so out of the ordinary that three separate former colleagues mention them unprompted. First he emits a deep gurgle for 30 seconds or so, then begins wiggling his hands and swearing under his breath in his broad northern accent: ‘Fuckingfuckerfuckingfuck’. The warm-up act has been known to last several minutes and has freaked out many a cameraman.

A friend recalls a conversation in early 2011, when Mason excitedly explained how he’d got in with an obscure group of Greek leftists called Syriza, who he predicted would soon be in government: ‘I’d never heard of them and thought, why are you hanging around with a party on two per cent? But Paul gets swept up, he loves the theatre of street politics.’

His Newsnight coverage grew increasingly focused on the apparent revolutionary fervour in Greece, which was famously challenged live on air by Jeremy Paxman: ‘Oh come on, Paul; it was hardly the entire population of Athens on the streets!’ Mason spent more and more time abroad, at one point refusing to film a dry economics piece in London because, he argued to his bosses: ‘I can’t do that; I’m a public intellectual.’

When he left Newsnight for Channel 4 News in 2013, his colleagues produced a private farewell video which teased him as ‘the economics correspondent who thinks he’s too grand to do economics’. Diplomatic editor Mark Urban, who may have felt a little aggrieved at Mason encroaching on to his patch, joked that he ‘doesn’t respect bourgeois constructs like job titles’. Paxman did the voiceover for a graph comparing the number of times he’d covered stories in Britain with the number filmed abroad. It was apparently ‘light-hearted but very cutting’.

At Channel 4, Mason was once again back and forth to Greece, flying business class and staying at the Intercontinental, the most expensive hotel in Athens. He struck up a close relationship with a filmmaker called Theopi Skarlatos, who he met in the middle of a riot and subsequently made two films with, including Love in the Time of Crisis. Thanks to the contacts he had built up he was granted extraordinary access after the 2015 elections. Mason became one of Syriza’s so-called ‘party journalists’, who act as a cross between reporters and party spokesmen in the Athens press. Friends say he now sees himself fulfilling the same role for Corbyn’s Labour.

In February of this year Mason quit Channel 4, complaining that the constraints of British broadcasting regulations had become too suffocating. The final straw, claim multiple sources, was the reaction of the Bank of England to a blog Mason wrote in which he accused Mark Carney of using ‘opaque and restrained language’ in a speech, offering his own interpretation of what the governor meant. Carney’s team made their feelings known and Mason resigned the same day. ‘His heart was somewhere else,’ suggests a former colleague.

Friends say Mason has been working under strict discipline for two decades and now wants to do the things he stopped himself from doing: ‘The filter has gone. He went from being an analyst to thinking, “This stuff is happening and I want to be involved in it.” ’ He has financial security from his book sales, a Guardian column and the £860,000 house he owns in south-London. Why It’s Kicking Off Everywhere sold 20,000 copies and Postcapitalism sold 60,000, generating revenue of over £1 million. If Mason secured typical royalties he will have banked a six-figure sum.

He was recently photographed in a snug leather jacket and critics mock him as being in the midst of a mid-life crisis. But he has always believed in Britain’s radical social democratic potential and sees Corbyn as the vehicle. He’s not a particular disciple of the Labour leader, say friends; he just thinks you’re either with Corbyn or you’re a ‘neoliberal’. He is close to McDonnell, having known the Shadow Chancellor for over ten years. When McDonnell tried to challenge Gordon Brown for the leadership in 2007, Mason was in regular touch.

These days he is in and out of McDonnell’s office far more than Corbyn’s, speaks with him and his staff several times a week and goes down the pub with them too. He is working with Momentum, the Corbyn campaign group, advising them to ditch some of their nuttier-rhetoric. What of the rumours that Mason might replace Milne as Corbyn’s communications chief? He strongly denies he will ever make his role official. The plan is for him to work, like he did in Athens, as a ‘party journalist’, advising behind the scenes and cheerleading on TV screens.

Make no mistake though, senior Corbynistas confirm he is acting as an aide in all but name. He believes Labour can do in Britain what Syriza did in Greece, and that his support for Corbyn will be vindicated just as his backing for Alexis Tsipras was when no one else had heard of him. Once asked to rate how satisfied he was with his life out of ten, Mason replied: ‘4.5. I don’t think I’ve yet done half of what I’m going to achieve and I’m a very restless person.’ Hold on to your Twitter feeds — you are now seeing Paul Mason unleashed.