'We need to lighten up a wee bit ... this is such an important debate'

Each morning, the CST, to use his Whitehall acronym, chairs a referendum meeting of officials and aides, who mull over the previous day's developments, including the media coverage, and plan the next move in the Battle for Britain.

As he stares out of his office window across the expanse of St James's Park, the Chief Secretary declares: "I'm in a unique position as a Scot in the Treasury with access to all the facts and information; someone who has a vote in this referendum and someone who cares passionately about the future of my country."

Mr Alexander is certainly at the sharp end, given the fate of Scotland and, indeed, of Britain will probably rest on the economic cases each side puts. The LibDem minister brushes aside Gordon Brown's charge that the Coalition has been guilty of positing a "Britain versus Scotland" case, saying that while one has to explain the merits of the Union, one also has to point out the demerits of independence. "It's been a balance," he insists, pointing out how the media will invariably focus on the negative as this is more newsworthy.

He complains about the "myths" put about by the Yes camp.

"Stuff about the oil revenues, for example. This notion was put around by an organisation called Business for Scotland - who ought to be ashamed of themselves, to be honest - that the British Government and the British taxpayer did not bail out the banks, it was the Americans and the Middle Eastern investors, who bailed out the banks. Stuff like that just isn't true."

But last month the SNP Government derided Mr Alexander and his Treasury chums, accusing them of "cooking the books" after Professor Patrick Dunleavy from the London School of Economics complained that they had misused his analysis to suggest the set-up costs of an independent Scotland would be almost £3bn. Indeed, Sir Nicholas Macpherson, the department's top civil servant later admitted that the public had been "misbriefed".

The Highland MP brushes the criticism aside, insisting Prof Dunleavy's numbers did not feature in the analysis the Treasury published; they did, however, feature in the pre-briefing of the Press. Rather, Whitehall focused on the more modest £1.5bn figure produced by Professor Robert Young of the University of Western Ontario. Yet he, too, pointed out that this was an extrapolation from other people's estimates.

Had Mr Alexander apologised to Prof Dunleavy? "I haven't spoken to him personally. I know officials here have had contact with him."

Had they apologised to him? The Treasury chief ignores the question and says what the whole episode showed was the people who want to set up an independent Scotland did not even know how much it would cost.

"It's an utterly extraordinary state of affairs. To hear John Swinney on Radio Scotland doing his Michael Howard moment when he was asked 13 times (and did not give a figure) was an extraordinary thing. That whole episode performed a service because it illuminated an issue, which is it's a jolly expensive business setting up a new country and the people who want to do it don't know how much it is going to cost."

Alex Salmond, who had also not produced a start-up figure, when he found out Prof Dunleavy had suggested £250m, declared this was a "reasonable" estimate.

"This would build a bridge about a quarter the way across the Forth. It's not plausible," declares Mr Alexander.

He insists the simplest way for the whole issue to be resolved is for the First Minister or the Finance Secretary to reveal to taxpayers how much it would cost to establish an independent Scotland. He claims they have the numbers but are simply not willing to reveal them.

"There is a lot of secret information still hidden within the Scottish Government. John Swinney's secret memo revealed they were doing a lot of work on this."

He refers to the reports that the FM has established a "transition planning division", accountable to a Director for Readiness.

"They now have whole teams of the Scottish Government basically doing contingency planning for the setting up of a new state," declares Mr Alexander. "They are entitled to use their resources how they want to but I would say they should be transparent with the Scottish people. If they are doing that work, what does it show? It's absolutely outrageous, to be honest. If they can't even tell us what a new state would cost to set up, I don't see how on earth all of their other deeply implausible figures can be believed by anybody either."

The Treasury numbers strategy came in for more stick when it was extrapolated how much the £1400 Union Dividend would mean to the average Scot. With the aid of Lego, it was suggested it could pay for "280 hotdogs at the Edinburgh festival" and watching Aberdeen play all season "with two mates; with a few pies and Bovrils thrown in for good measure".

While the Yes camp upbraided the Treasury for peddling patronising nonsense, Mr Alexander insists some people had suffered a "colossal sense of humour failure".

He goes on: "This was something prepared for a particular social media channel. It may not all have been particularly perfectly worded but it was a perfectly reasonable attempt to be slightly light-hearted about this.

"God knows what Scotland would be like if all these po-faced Nationalists were in charge of an independent Scotland, there would never be a joke told anywhere.

"We need to lighten up a wee bit because this is such an important debate … It is incredibly serious but a wee bit of humour never hurt anybody."

Mr Alexander admits that sometimes such things "misfire" but denies the £1400 Union Dividend point was undermined.

He then points out: "Of the £1400 a year, £4 is accounted for by a start-up cost of a new state. So the only part of our analysis that anyone has tried to challenge is £4. That means £1396 of the annual UK Dividend has been unchallenged."

He goes on: "If you look at all the different elements that go into the £1400, the set-up costs when distributed across the whole population, shared out, the estimate we have given is £4.

"So £1396 is unchallenged by the Nationalist side. Well, I'm willing to have the debate about whether it is £1396 or £1399 or £1402 but the order of magnitude is clear."

Yet, when expressed in those terms, set-up costs of just £4 per person might not seem such a bad deal to some pro-independence Scots.

The Treasury Chief stresses that all the Coalition numbers plus the analysis from think tanks like the Institute for Fiscal Studies over the last few weeks conclusively prove the "bottom has fallen out of the Nationalist economic case". He lambasts the Scottish Government's predicted oil revenues as "fantastical…pie in the sky stuff".

Asked, in the event of a Yes vote, if the geographic share that would give Scotland 90% of North Sea oil and gas was a given, Mr Alexander describes it as a "perfectly reasonable working assumption," but adds: "Of course, it would have to be part of negotiations."

He again insists a currency union is a non-starter and would for Scotland not mean independence as the government in Edinburgh would have to "bind its hands on tax and spending", handing control to London, adding: "It's a bit like I'm going to try and sell you a car and I say unfortunately I have had to remove the steering wheel. Well, that's not much of a car. When the road Scotland follows turns one way or another, you would not be able to follow it."

Needless to say, the Inverness Liberal Democrat welcomed the recent intervention by the US President, suggesting he wanted the UK to remain united.

"I agree. In Scotland we often say, what's the Burns phrase? 'O wad some Pow'r the giftie gie us, to see oursels as ithers see us.'

"It's important for people, in reflecting the choice we're going to make, to think about what does that mean for our standing in the world, what does that mean for our historic alliances, what do other people think about this?

"Barack Obama certainly wasn't trying to tell us what to do because he was clear: this is a decision for the folks in Scotland. But sometimes the Nationalists pretend, it'll all be fine, we will be great leaders on the world stage, etc, etc.

"To hear the President of the United States say that the UK has worked pretty well, he wants it to be strong and united, I really believe this. Strength in unity is something that gives Scots much more international reach and an ability to shape what's going on in the world than we would ever have as an independent country."

Despite all the campaign's passion and negativity from both sides and even the occasional name-calling, Mr Alexander is surprisingly upbeat, believing the referendum has re-ignited Scots' interest in politics.

"What it has done has created an opportunity to go back to an older style of politics, which is a good thing. The late Russell Johnson, who was a wonderful man and one of my inspirations, was telling me when I was selected for my seat that in his early elections his entire campaign had been public meetings. He had done 70 or 80 over three or four weeks. You would get scores or even hundreds of people turning out because they wanted to hear directly from the person, they wanted to ask them the difficult questions and all that.

"One of the things I have enjoyed about the campaign so far is, if I turned up for an 'ask the MP' session, I would probably get a few folk, I wouldn't get the 80 or 90 at the Better Together public meeting."

Indeed, when I suggest the passions involved in the debate could poison the well of Scottish politics, the Treasury Minister disagrees strongly and feels it could do exactly the opposite.

The re-engagement of people to politics is a "moment of renewal for our country", he insists.

"I don't think it should have a divisive legacy, it should have a unifying legacy; that whatever we decide, we have decided this together. We've all had our arguments in our own homes, in the pubs, in the television studios and in newspapers, and we've made this judgement and we've come together and we all respect each other's opinions.

"Provided we all conduct the debate in a civil tone, there's absolutely no reason why this shouldn't be a moment of rebirth for Scotland as opposed to a moment of recrimination."

Asked, following Scotland's renaissance, if he would one day like to succeed Nick Clegg and climb to the top of the Liberal Democrat tree, Mr Alexander pauses and gives a classic political answer.

"I've always said I have a big job to do, that consumes all of my attention. Nick and I are close friends, I have been a strong supporter of his; I helped run his leadership campaign, for goodness sake, when he first got elected.

"I want him to carry on for many years to come and so at the moment the obligation on me is to make the argument for the fact that the recovery would not be happening without the Liberal Democrats. That's more than a big enough chunk to have bitten off for the time being."

That's a Yes then.

Given much of the world is gripped by soccer fever, I ask that question - as a proud Scotsman will he be supporting England in the World Cup?

Ever the Unionist, he replies diplomatically: "I would like to see England do well; of course I would as a UK team as it were."

He then adds with a loud laugh: "But I can tell you I have drawn Germany in the office sweepstake."

England or Germany? It's a no-brainer.