In John Swinney’s office inside the Scottish Parliament in Edinburgh there is a piece of stained glass. It’s the saltire with the inscription: “Now is the day, now is the hour”. The words are from Robbie Burns, imagining Robert the Bruce’s eve-of-battle speech at Bannockburn in June 1314:

Now is the day, and now is the hour:

See the front of battle lower,

See approach proud Edward’s power

– Chains and slavery.

Who will be a traitor knave?

Who will fill a coward’s grave?

Who’s so base as be a slave?

– Let him turn, and flee.

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In a vase, there are more Scottish flags and thistles. On a shelf there are souvenir bottles of Scotch, and pictures of Swinney meeting various delegations of overseas visitors. And on his desk is a copy of Scotland’s Future – Your Guide to an Independent Scotland.

In the Scottish independence movement only one person is above Swinney, and that is Alex Salmond, the SNP leader. Ever since he was 15, when he joined the party, Swinney has been pursuing a “free Scotland”.

He’s now 50, having risen to the top post before being succeeded by Salmond in 2004. He’s the MSP for Perthshire North, having previously been MP for Tayside North in the House of Commons. But the real interest in him lies in the position he now holds as finance minister – the man who would be George Osborne should the Yes vote triumph – “Honest John” Swinney, charged with advancing Scotland’s economic fortunes.

Swinney is mild-mannered, softly spoken, more studious headmaster than tartan-clad warrior. He studied politics at Edinburgh University and the cause of Scotland has been at the heart of his career.

By way of greeting, I ask him how the great struggle is going? His smile is thin, but it’s definitely a smile. “Over the last eight to nine months we’ve seen a meeting of the polls. It was 52 per cent No and 48 per cent Yes at the weekend, and we’ve also had a 55 per cent and 45 per cent.

“The crucial thing is that the gap is narrowing. There’s a twofold cause. One is the publication of Scotland’s Future, which sets out a clear argument for independence. The other is that as the referendum gets nearer more people are engaging in the debate. I’m confident we will win – we’re seeing plenty of evidence of the closing of the gap over the last few months.”

I tell him that my taxi driver said he could not vote SNP because he had little faith in Scotland’s economic ability to stand alone. “Your taxi driver is right,” Swinney says, before pausing to make me think he’d undergone a Damascene conversion. Not a hope. “It will turn on money, and where people see their best hopes of economic prosperity. They will realise what our opponents are saying – that they will be worse off if Scotland is independent – is a lie. In the end, it will come down to their judgement as to where the best prospects for families belong.”

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What puzzles me is the currency question – why he’s not going down the same route as other small countries with their own currency – Denmark with the krone, say, or Ireland with the punt. In the latter’s case, in particular, the similarities with Scotland were close.

He’s heard the arguments for and against, he says, “and academics, including two Nobel laureates, and the Fiscal Commission Working Group, have looked into the variety of options available and in their view the preferred option was for a sterling zone between an independent Scotland and the rest of the UK.”

The main downside to Scotland having its own currency is “the transaction costs which will run at £500m a year.” That’s how much businesses south of the border will spend changing currency to trade with the north. That will be saved, plus “the sterling area will benefit from the economic activities of Scotland, from the contribution it makes from things like whisky and oil”. Financial services will also operate more smoothly across Scotland and the rest of the UK, he maintains, if they share a common currency.

But wait a minute. Haven’t Standard Life and other Scottish financial firms said they might quit Scotland if it votes Yes. “I’ve spoken to Standard Life and they’re fine. With the sterling zone comes financial regulation across the whole of the sterling zone area and they’re fine with that.”

But how can Scotland be truly independent if financial regulation is run out of London and sterling is effectively controlled by the Bank of England, via the setting of interest rates and the Bank’s Sterling Monetary Framework? That doesn’t sound very independent.

It will be independent, he maintains – it’s just that financial regulation and monetary control will reside with independent institutions, as they would if the Scottish government created its own regulator or Monetary Policy Committee. There’s no difference. But they would be in London, not Edinburgh.

OK, so what about Scotland risking becoming another Iceland, with its GDP far outweighed by its banking sector? I’m reciting pro-Union propaganda, apparently. “The Treasury inflates the size of financial services in relation to the Scottish economy. For instance, the international section of Royal Bank of Scotland has always been headquartered in London.” And since the crash, the “whole thrust of international banking reforms is based on where the banking activity takes place, of making the people there accountable and regulating the activity there.”

So a bank could be headquartered in Scotland, and to all intents and purposes would be Scottish, but the Scots could then disown some of the things it does? He does not really take the point, preferring to dwell on the “safety blanket” effect of being inside the sterling zone, and the stable business environment that could create.

Later, Swinney’s assistant sends me a note on bank bailouts which points to evidence that in future no country would be expected to rescue its banks on its own, that these are international banks and the international community must respond collectively. It quotes Professor Andrew Campbell of Leeds University: “It would be inconceivable that Edinburgh or Scotland as a whole could be held liable for that full bill. I know we’re talking about a scenario which doesn’t actually exist, but I really don’t think Scotland could be held liable in that situation.”

I’m getting a picture of a “pick’n’mix” form of independence – a country that would take the rap for some things but not for others, that would bask in the pride of its banks but not pick up the tab, which would take its cue from another country in terms of setting interest rates and financial regulation.

Pulling the two countries apart is not going to be easy, I suggest to Swinney. How, for instance, would he deal with the DVLA? Would every car owned in Scotland have to be re-registered with some new body? “There’s a route map through those issues,” he says. “In some circumstances there’s an argument for continuing to share services. We will do it if suits us to do so. My view of independence is that it would not matter if we shared something like the DVLA with the rest of the UK.”

Eh? That’s not very independent either, is it? Not so, he argues. Already around 60 per cent of public services are devolved to Scotland. As for the rest, “there will be a need for a transition period while we work through the arrangements.” That may entail separation or Scotland continuing to use them – it’s not yet been decided.

So what about the EU? Would an independent Scotland be in or out? “We’ve had a two-year debate on this. An independent Scotland will be a member of the EU.” Even though the EU’s President, Jose Manuel Barroso, has expressed misgivings about allowing Scotland to join straight away? Swinney shakes his head. “It’s interesting, the degree to which others within the EU are not absolutely behind the comments he made. They weren’t exactly forthcoming in their backing for him.”

For more than 40 years, Swinney asserts, Scotland has been part of the EU. It already meets all the requirements for membership. “Its legal framework is entirely compliant with membership of the EU.” And besides, he says, there is no mechanism for expelling any existing part of the EU.

Yet again, this does not seem very independent, nor very proud, nor indeed very thought-through – coming up with arguments as to why something cannot happen rather than Scotland taking charge of its own destiny and demanding membership in its own right.

Swinney remains confident that an independent Scotland would attract inward investment. He had been worried, he admits, lest uncertainty about independence deter someone from investing. But he maintains that this has not happened. “The Ernst & Young inward investment attractiveness index shows that Scotland is the highest performing part of the UK apart from London. The debate about independence is not harming Scotland’s ability to attract investment. Tourism spend is up 20 per cent this year. The world’s attention is on Scotland and that is a very significant plus.”

Swinney’s aim is to build a dynamic economy to boost public finances. “We want higher employment, lower unemployment, lower youth unemployment and lower female unemployment. We can achieve that as a direct result of our intervention in restructuring the economy.”

So, his Scotland will: provide free childcare in certain categories in order to increase female employment (paid for, he says, by removing Trident from the Faslane naval base that costs the Scottish taxpayer £900m a year); invest heavily in infrastructure; maintain an open-door policy towards foreign students; and increase R&D (“our private sector’s spending on research and development is one-fifth that of Finland’s”).

He’s on a roll now. Suddenly, however, he has to stop – his aide is signalling frantically. He’s got to rush off and vote. I’m left looking at the inscription: “Now is the day, now is the hour”. After 700 years, Robert the Bruce’s struggle is about to be finally decided.

The CV: John Swinney

Born: 13 April 1964, in Edinburgh

Educated: Forrester High School, and Edinburgh University. Studied politics

Non-political career: research officer, Scottish Coal Project; strategic planner, Scottish Amicable

Political career: joined SNP at 15; National Secretary, SNP; MP for Tayside North; SNP leader; MSP for Perthshire North; Finance Secretary in Scottish Government

Family: two children from his first marriage, which ended in divorce. Now married to BBC journalist Elizabeth Quigley, with whom he has one son, born in 2010.

Won’t be allowed to forget: on the BBC’s Good Morning Scotland was asked 13 times by presenter Gary Robertson to put a figure on the start-up costs of an independent Scotland, and refused