ALEX Salmond bursts into the single quiet room within the SNP's whirring headquarters, pulls off his tie, and flops into a window seat with the look of a runner coming off an exercise rush, tired yet strangely euphoric.

It's been marathon after marathon for more than two years, and it's still not over, yet he seems bright, chipper and remarkably confident.

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What has he learned entering the last leg?

"Oh, the country has changed. The mood of optimism, confidence, participation, engagement. Somebody should write a thesis some time about what this is doing to the health of the nation. I've never seen more optimistic, happier people -in places where people are often neither optimistic or happy - in my life."

He says he's also learned a lot about the No camp in recent weeks, as the polls narrowed and Unionist nerves starting showing the strain.

"I'm surprised by the total disarray in the heart of Westminster ... by just how unprepared they were for the poll reversal they had, which I always anticipated. Win, lose or draw - and I think we'll win and a draw's unlikely - they have fought a dismal campaign. They've lost huge sections of people who vote Labour.

"Whatever happens to the Scottish political firmament, the Labour leadership are in real trouble. Change is afoot. Something has been stirred here which is not going to going away."

He says Better Together also has a fundamental problem in that what unites the three parties is a dislike of Yes, rather than anything positive. "Their unity is based on a negative," he says. "It's not a winning force in politics. At the end of the day, people vote for things, not against things. It's a lesson I learned some time ago, and I've yet to be proved wrong."

But Better Together will pitch a No vote as a vote for the Union and for more powers, won't they? He scoffs. "What we're being offered is a spatchcock, last-minute commitment to a timetable to a discussion based on something so flimsy it's embarrassing even to put it on one side of a piece of paper. It makes Alec Douglas Home's offer [of "something better" after a No vote in the 1979 devolution referendum] look magisterial in comparison."

He's just come from a tetchy media briefing at which he accused the BBC's political editor Nick Robinson of "heckling" him.

Is the BBC's referendum coverage biased?

"Yes, absolutely," he says. "Of course it is. The problem with Nick … I mean, don't get me wrong, I like these folk, but they don't realise they're biased. It's the unconscious bias which is the most ­extraordinary thing of all. If the BBC were covering, in my estimation, any referendum, in any democracy, anywhere in the world, they would cover it impeccably, in a balanced fashion.

"What they don't understand is they're players in this."

He says BBC journalists from London are reporting old news as fresh out of ignorance.

"The coverage of the last few days, for example, the whipped up metropolitan media coverage, 'My God, Bob Dudley [BP's chief executive] has commented! Standard Life has issued a statement!' For the metropolitan BBC journalists this is extraordinary news.

"I'm not really laying this charge at BBC Scotland. I just think metropolitan BBC has found this whole thing extraordinarily difficult, to separate their own view of the world from their view reporting Scotland."

And that has translated into bias against the Yes campaign? "Yes, absolutely."

He later claims the bias won't matter as "the truth is out there" on social media, but he sounds thoroughly miffed.

The questions that riled him were about banks heading south because they were worried about bail-out protection and the currency. Despite the Unionist parties saying there won't be a formal currency union with an independent Scotland, Salmond says a "commonsense plan for a common currency" will prevail.

It's a nice soundbite, but as the economist Michael Dooley pointed out: "Monetary arrangements are given birth at conference tables, and laid to rest in foreign exchange markets." Why should markets trust a currency union that one partner doesn't want?

"What are they going to do?"

Markets are inventive. If they could make money betting against a currency union, they would -wouldn't they?

"Capital movement depends on a redenomination risk [ie your money becoming a new currency], and we've made it abundantly clear we're having sterling. We can't be stopped from having sterling. There is no re-denomination risk."

Salmond has cited the long-lived currency union between Luxembourg and Belgium as a precedent. But that was based on the two countries signing an initial 50-year treaty commitment. Is that what's needed to share the pound?

"I think that's a matter for negotiation. In respect of the currency union, if it's agreed," he catches himself, "as I'm certain it will be incidentally, it will be recognised that the attacks on it were campaign posturing. There is no reason for anybody to believe other than that this is a stable currency union."

But if there's an irresistible economic logic to it, why has he got three Plan Bs?

"Because the Fiscal Commission Working Group, very fairly, was asked to examine the range of monetary policy options and come to a recommendation."

He makes no mention of the fact that he was the one who asked the group to look at other options.

"But this argument's been won," he carries on, "for the following reason: that the overwhelming majority of people want to keep the pound, and secondly they don't believe the Westminster politicians."

However, he is describing a campaign argument. It doesn't mean his argument with the rUK has been won, or market scepticism has been overcome.

Oil has also loomed large in the campaign, with varying estimates of the reserves and tax revenues an independent Scotland might enjoy.

How much does he think is in Scottish waters?

"I think we're perfectly entitled to use the Oil & Gas UK estimate of up to 24 billion barrels."

The International Energy Agency says two-thirds of the world's fossil fuels should be left untouched to save the planet from catastrophic climate change, or we'll all cook. How much of Scotland's oil should be left under the sea? Despite his Yes Scotland alliance with the Greens, there's nothing green about his answer - he wants to use every last drop.

"I think we should develop our natural resources to the fullest extent. And we should develop a method of getting carbon dioxide back under the sea in aquifers. The IEA says many interesting things, but I think we should develop."

The referendum has been thick with conspiracy theories, mainly on social media, many starting in the Yes camp. We rattle through a few.

Is there a massive unreported oil field off Shetland under the Clair Ridge?

"There is a lot of suggestion."

Is it credible suggestion?

"Oh yes, it's credible. But can I confirm it? No. What I can tell you is many of my constituents believe this to be true."

Are they misguided?

"Well, I don't know. I can't say for definite. There are suggestions that BP and Total have been excited beyond belief at some readings. Can I say that I know this information is being held back, suppressed deliberately until after the referendum? No, I don't know that. But is it a credible idea there's potentially lots more oil off the west coast of Shetland? Absolutely."

Jim Sillars, his former SNP deputy, believes MI5 are working to secure a No vote. Does he agree?

"Well I haven't met them thus far."

He pauses and leans forward with a grin.

"At least as far as I know, Tom."

Are MI5 involved or not? "I've no idea."

In one recent poll, 26% of people thought it was "probably true" that they were involved.

"Look, Jim's been around politics a long time. I don't discount anything he says. My own view is that, whether or not that is true, I don't think it has much effect, and therefore we should just proceed in our campaign and get on with it."

The same poll found a fifth of people thought the ballot might be rigged. Does he agree? "No, our returning officers and police service will ensure a proper ballot in Scotland."

If there is a Yes vote, Salmond says he will assemble a Team Scotland to negotiate the terms of independence with London.

He says academics and experts are signed up to help with the fine detail, as well as groups from civic society. Salmond himself would lead the third "layer" of the team, the politicians.

He's full of praise for fellow Yes campaigners Patrick Harvie, the Green MSP ("I'm enormously impressed with him") and Scottish Socialist Party co-convener Colin Fox ("a star").

But they want an end to the monarchy, a new currency and to be outside Nato. Would they be in Team Scotland?

"Yes, of course. They're part of the Yes campaign. They're entitled to be there. Remember, this is not going to be an argument about having a programme for an election in the spring of 2016. This is about negotiations to get the best possible settlement for Scotland."

Fox and others on the left see the referendum as fundamentally a class struggle. Does he?

"No," he says. "That's their prerogative."

And the polls show a clear division between the affluent ABC1s and the working-class C2DEs, with the later mostly for Yes and the former for No.

"That doesn't make it a class struggle. That just means different groups in society, as yet, are reacting in different ways to the campaign."

Will the middle classes be shocked if the working classes deliver a Yes vote?

"I think people in Scotland will rejoice if there's a Yes vote."

He mentioned the health of the nation earlier. What about his own health through the campaign?

"I'm delighted to say it's absolutely superb."

Because there have been rumours at Holyrood of heart trouble recently.

"No. I don't have any heart trouble."

And that his much-publicised 5:2 diet is related to heart trouble.

"No, nothing. Absolutely nothing to do with it. I think the only flutters that are taking place are in the bosom of the No campaign."

He's always said the result would be a Yes. What about the scale of the victory?

"I'd be absolutely delighted with 50.1%. I do have something [greater] in mind but I'm going to leave it to the people."

Is he in any doubt it's going to be a Yes?

"I am as confident as I possibly can be that something fundamental has shifted in Scottish society and that's going to shift us to a Yes vote. Whatever barrage comes at people over the next few days, I think it's too late."