The SNP wanted to freeze his Scottish Socialist Party (SSP) out of the fledgling Yes movement. One SNP MP told Fox there was simply "no place" for the far-left group he had led since 2005. At the time, the SNP had a point.
Formed in 1998 with Tommy Sheridan as leader, the SSP was an early phenomenon of devolution, with Fox and five other MSPs elected in 2003.
But 18 months later, Sheridan's resignation amid a sex scandal led to a catastrophic split and ultimately an electoral wipeout.
By the time the referendum became a reality in 2011, Sheridan was in prison for perjury and the SSP was a ghost party, its most solid quality a reputation for vendettas.
Its vision of Scotland as a socialist republic outside Nato with a new currency didn't endear it to a cautious SNP leadership either.
However, Yes Scotland chief executive Blair Jenkins ensured "The Trots" were brought aboard. It was a turning point, says Fox, who is still grateful to Jenkins and nippy about the SNP.
"The SSP has been brilliant for the Yes side and the Yes side has been brilliant for the SSP," he says. "It's put new life in us and new blood."
While they don't have much cash, the SSP offer Yes Scotland something more precious. Like the Radical Independence Campaign, they help the Yes camp reach out to the "missing million" - the working class, habitual non-voters who could decide the result, but whom mainstream parties have ignored and lost touch with.
"What the SSP brings is a sense of the schemes, the workplaces, the unions," says Fox.
"The SSP has brought a proletarian sense to Yes Scotland and reminded people the decisive issue is whether people think they're going to be better off. It's not the currency, it's not the EU, it's not those highfalutin' chattering class issues. It's about whether people are going to say, 'I'm better off'."
If it is a Yes vote, the former Lothians MSP predicts the largely No-supporting middle classes are in for an almighty shock.
"They'll be horrified," he says with approval. "The middle classes in Scotland are complacent. They pay lip service to shocking poverty and appalling inequality. They say they care. But when it comes to [fixing] it, it's 'Oh, I'm not paying any more taxes. I'm not giving money to these junkies, these benefit scroungers, these immigrants.' You scrape the surface and they're not as progressive as they think they are.
"If you're going to redistribute wealth in Scotland it's like an omelette, you have to crack some eggs. Somebody has to pay more."
It's very different from the SNP's pitch.
"The SNP are fundamentally a capitalist party which wants to placate the big corporations who run this country," Fox goes on. "They're not in favour of redistribution in any meaningful way."
As for the SNP plan to keep the Queen under independence, he says: "A feudal monarchy in 21st-century Scotland? It's farcical."
He continues: "You have to be absolutely clear that independence represents change. It's a profound change. You can't say independence is just the status quo - we keep the Queen, we keep the pound, we keep Nato, we keep the banks in charge, we keep things as they are."
Yet, despite the differences, Fox says the SSP are ready to work alongside the SNP because they share the same goal of a Yes vote.
However, after that it might get rocky again.
Fox believes the SNP White Paper is a "contribution" to a post-Yes Scotland but "shouldn't necessarily be seen as the final word on it".
And to ensure independence isn't just what the SNP want, he is insisting that the SSP would be in the core negotiating team with London.
On the pound, Fox expects there to be a currency union, but only for the 18-month negotiation phase up to Independence Day in March 2016. After the negotiations, he reckons Scotland will have a new separate currency.
"That would be my expectation," he says, adding others in Yes Scotland are of the same view. Pressed, he says September 18 will be "a close Yes", but doesn't sound terribly convinced.
However, Fox adds that the prospect of another Tory government is a "huge catalyst in the schemes".
But didn't he say those folk are disengaged?
"They're not so disengaged they don't know the Tories are a***s in Scotland. Jesus! They're no' blind, stupid and deaf, you know."
If it's a No, "the struggle continues", he says.
One option would to be to seek a mandate for independence via the 2016 Holyrood election.
"If, in 2016, the SSP, the Greens and the SNP get a majority, the very next day we're all on the first train from Edinburgh to David Cameron."
Battle-weary voters may be less enthusiastic.
Meanwhile, no interview with Fox can avoid Sheridan. Is his old enemy making a referendum comeback? "I couldn't care," he says. "Tommy Sheridan is the Lance Armstrong of Scottish politics. The only difference is that Lance Armstrong finally admitted what he did.
"Poor Tommy, he's not offered Scottish politics anything in a decade other than his court case. He's finished. He's been finished a long time."
Against a tongue so sharp, David Cameron might want to dust off a family suit of armour for those post-Yes negotiations.