'Other MSPs and members of the SNP send their kids to private school. I don't think it's fair to pick out one person. But as a matter of principle I wouldn't do it ...'

"We're winning the argument. But I wouldn't underestimate what's going to happen in the next seven weeks."

Curran knows the doomsday battle ahead is for the support of the million-plus Labour voters who backed Gordon Brown in the 2010 election, and who, because of their sheer number, have been targeted relentlessly by the Yes campaign.

If Labour can't hold on to them, it will lose the referendum, faces being replaced by the SNP as the party of the centre-left at Holyrood, and would probably be cast into the abyss at Westminster by England's electoral arithmetic.

Curran's job is to renew the party's bonds pronto. Her doorstep pitch has three strands, starting with the magnitude of the decision.

"It's irreversible, forever ... people get that.

"Then you say: 'If we're going to do this, here are the questions we need to ask ourselves' and largely that's about the economy.

"Then it's for us as Labour people to say: 'We want to change our country, we want to make our country a better place. This isn't the way to do it. This would set us back on that journey. There's a better way to do it.'"

Despite some polls showing around one-quarter of Labour voters planning to vote Yes, she claims that doesn't tally with her own experience.

"I understand the resentment that people have about their conditions, and about how they want things to improve. But I'm not seeing great swathes of Labour voters persuaded by Yes."

What about the Labour for ­Independence group? "They're not real," she snorts.

She predicts the decision people make on September 18 will be a nuanced mixture of head and heart, but one factor will loom large.

"Scots will never vote against their own best financial interests. I don't think that's who we are. You can't ask people to be irrational.

"But what you must do is make sure people feel good about the choice they're making, and it's the right decision for themselves, their family and their country.

"We've got a confidence and an energy about taking that argument to people."

Does Alex Salmond come up much on the doorstep? "He comes up a lot. Very negative.

"People believe he is so blinded by his dedication to independence that he's thinking about that rather than their interests, and voters always expect you to think more about their interests."

She attributes the stubborn gender gap in the polls, with women more likely to vote No, to their feeling they have "a wider responsibility", that it's not just for themselves.

"They have to think of their family, children, grandchildren and wider community. The SNP have singularly failed to have a message to women that works.

"To have a childcare offer that they could do right now had a cynical feel to it.

"Women saw the SNP as trying to win women's votes rather than change women's lives. People saw it as a chat-up line rather than an in-depth commitment."

A desire for social justice has been the dynamo driving Curran's politics since childhood.

As she knows, Scottish Labour's deputy leader, the Glasgow Central MP Anas Sarwar, has sent his son to £8234-a-year Hutchesons' Grammar.

Do private schools further social justice? She says she's anticipated the question, but still looks uncomfortable.

"Other MSPs and members of the SNP send their kids to private school. I don't think it's fair to pick out just one person. Joan McAlpine sent her kids to private school. She's a very senior SNP person. That's for them to decide.

"I'm clear about where I stand and I'm clear about Labour's commitment to education in the state sector. That's our absolute priority."

Did it ever cross her mind to do it?

"No, never. As a matter of principle I wouldn't send my children to private schools."

Curran's oldest friend in politics is her fellow Glasgow University alumnus Johann Lamont, who the Holyrood gossip mill predicts will quit as the Scottish Labour leader before 2016.

Is Lamont happy as leader?

"Yes, she is. She's very focused and clear."

Does she have a hunger to be First Minister, or is she half-hearted like Iain Gray?

"Yes, she absolutely has. Johann would be a first-class First Minister. She's guided by what's best for the Scottish people ... not her own personal ambition.

She wouldn't have stood for the job if she didn't want to do it."

So she'll still be leader at the 2016 election?

"Yes, I'm convinced of that."

Will Curran be glad when the referendum's over? "Yes and no," she laughs, relaxing slightly. "It's actually been really interesting."

She reckons that, besides a No vote, there's another twist lurking round the corner for the SNP - a revived Labour ground operation crossed with renewed public engagement in politics.

"When you go to a door, people want to talk to you. They're not saying 'you're all the same'.

"See the material you get out of that, the data we've got, the interest we've got!

"I have learned so much from this. I feel more confident in my politics than I've ever been.

"The SNP might rue the day they gave us such an opportunity to talk to the Scottish people."