Darling: we need to win by a respectable majority because it would be good to put this to bed

It has not been a good day for Better Together. YouGov, a pollster that has tended to find lower levels of support for independence than its rivals, has just put the gap between the two camps at a mere six points. To put that in context, Better Together was ahead by 14 points three weeks ago and by 22 points at the beginning of last month.

Then, in a poll at the weekend, YouGov would go on to put the Yes camp in the lead for the first time.

The most confident campaign insiders are talking of "holding their nerve". The jittery are now thinking the unthinkable: they might lose.

The famously unflappable Mr Darling, who has led the cross-party campaign since it was set up two years ago, is not in the latter category.

He says: "Earlier this year people would say to me, 'It's in the bag, what have you got to worry about?' I always said then, as I say now, 'No it's not.'

"What you are finding is that people are still tornand it's our job to try to get them over the line. There is a lot of volatility around with people going between No and Yes, Yes and No."

The former Chancellor, like others in the No camp, firmly believes that support for No will harden in the privacy of the polling booths, when people contemplate the enormity of the decision they are being asked to make. He adds: "I think it's all part of the fact the stakes couldn't be higher.

"A lot of people when faced with the ballot paper will realise what a dramatic choice they are making.

"It's going to be close and your vote could make the difference.

"If we decide to leave by one vote, that's it. There is no going back, this is not like a general election.

"You can't give this a try because, if you vote for independence, there is no going back. If it turns out that Alex Salmond is wrong on any number of things, the rest of us will pay a very heavy price."

Mr Darling, the man who piloted Britain through the banking crash of 2008, says his confidence is based on concrete campaign data.

As the race enters the home straight, he says, Better Together is running at full speed.

"We've more organisers now than I would have dreamed of a year ago. I see our daily canvass returns, we are doing the phone calls, making the contacts and my parliamentary colleagues, MPs and MSPs, are all out every night and every weekend.

"It's making a huge difference and, yes, we'll get our vote out."

As part of the extra effort, Mr Darling shared a platform with Gordon Brown at a recent rally in Dundee, ending a rift dating back to their time in numbers 10 and 11 Downing Streets.

The former Chancellor, who resisted the former Prime Minister's attempts to sack him in 2009, looked as relaxed on that occasion as he does in his office.

The only visible change two and a half years of referendum campaigning has made on him is a new pair of specs, the legacy of a TV debate mini-makeover.

Beneath the calm exterior, however, does he not fear, as the polls narrow and Yes Scotland's impressive on-the-ground operation steps up a gear, the momentum is with his opponents?

"No I don't," he says flatly. "I think opinion polls are opinion polls. We have momentum, our vote is still increasing. I'm confident we will win but we need to win by a respectable majority because it would be good for everyone if we put this issue to bed."

What's a respectable majority?

"You'll know it when you see it."

He doesn't believe the leaders' TV debates, for all the headlines they generated, will make much difference. The first, on STV, he was judged to have won narrowly, the second, on the BBC, he lost comprehensively to a rejuvenated Mr Salmond, according to most commentators. He says he has not lost sleep over either.

"I think the big question you ask yourself is does it make any difference to the way people are voting," he says. "I actually think that having two men shouting at each other doesn't do anybody any favours whatsoever.

"I read somewhere that something like four out of 10 people never made it through the entire programme.

"As I say, for most people who are not politicians or commentators, and who are looking at this and thinking what should I do on the 18th, I'm not sure it will have made any difference. People are listening far more to their friends and family than they are to politicians.

"Let's be realistic: most people can take or leave politics and preferably leave it. They listen to what people have to say but at the end of the day they make up their minds using their own common sense.

"It's common sense in the end that will win the day."

The confidence and sense of calm shared by most within No campaign has been striking. However the gradual erosion of its lead, a phenomenon reflected by different polling companies has raised questions about its strategy.

Professor John Curtice, the country's leading psephologist, has argued that one of Better Together's key messages - that Alex Salmond's proposal for a currency union with the rest of the UK is dead in the water - has failed to hit home with voters. What's more, throughout the campaign the SNP has successfully portrayed the No side as negative.

Recent billboards designed to tug at voters' emotions, with slogans including "I love my family, I'm saying no thanks", have been widely criticised. A broadcast aimed at undecided women voters was condemned as "patronising and sexist".

Mr Darling is adamant, however, that Better Together has taken the right approach.

Defending the campaign's advertising, he says simply: "When your opponents attack your advertising there is usually a reason for it.

"Right from the start I said we'd set out a positive case for us to be part of the UK, principally on jobs and opportunities - there are a million jobs dependent on us being part of the UK - and on the security we get, on pensions and if there is a shock to the system like the banks a few years ago. But I also said it is the Nationalists who are putting forward a proposition. This isn't a contest between political parties. They are putting forward a proposition that we should leave the United Kingdom.

"No-one was going to stop us from asking hard questions.

"When we asked, where is the legal advice that shows we automatically get into the European Union, it turns out that not only did it not exist, it had never been asked for.

"Similarly, when the Institute for Fiscal Studies said we'd face a £6 billion funding gap, which would have to be funded from cuts or tax increases, you are entitled to put those questions.

"'Negativity' in this campaign has been asking Alex Salmond a question to which he does not have an answer.

"You're asking us to take a fundamentally different direction, you are asking us to do something completely different. You'd be naive if you didn't ask questions. I'm not making any apologies for that. How on earth could you go through a campaign without asking any questions?"

Mr Darling rejects the notion the Better Together has blown itself out. Rather, he's angry at what he claims has been a concerted Nationalist attempts to close down questioning and undermine critics.

"For most of my political life the Nationalists have dealt with criticism by abusing whoever asked the questions.

"They have always played the man not the ball," he says.

"It's the oldest trick in the book. Don't answer the question, just abuse the questioner.

"But they are not getting away with it. You have to bear in mind, they are behind in every single poll.

"Whether it is relation to the currency, to Europe, to welfare, to pensions, you name it, you know when they are in a hole they start abusing anyone who asks the question.

"Look at the problems we have been having on the internet. Anyone who questions them is subjected to abuse.

"What sort of country would that be. What sort of country would Scotland be if this is what happens to anyone who questions the government of the day? I just don't think it's acceptable."

Mr Darling is equally scornful of Yes Scotland's focus on the NHS and its claim that privatisation of health services south of the Border will have a damaging impact in Scotland.

NHS Scotland will be starved of funds or forced to privatise more of its own services, Yes campaigners have warned.

They've been accused of spreading "the biggest lie of the campaign" by Labour yet the message appears to have contributed to the Yes campaign's surge in the polls.

Mr Darling says: "After the first televised debate it was obvious they were in trouble with their economic arguments and they would come along with something else.

"It's striking that in the first debate Alex Salmond mentioned the NHS once. In his draft constitution for Scotland published in the summer it wasn't mentioned at all.

"They've now come along with all these scare stories.

"When you look at what they said in their manifesto in 2011 they said they could protect the Scottish NHS because it is devolved.

"The only person who can privatise the health service in Scotland is not the Prime Minister, it is the First Minister."

He adds: "Nothing surprises me, frankly, I think their view is anything goes.

"Health is totally, 100 per cent devolved.

"The Scottish Government can spend as much or as little as it wants on the health service."

He rejects utterly the idea that health spending is set to reduce in England, a move that would cut Scotland's budget under the Barnett Formula for distributing public spending around the UK.

He insists: "Affection for the NHS stretches right across political boundaries.

"Even Mrs Thatcher in the 1980s, when she looked at an American-style privatisation, took one look at it and thought: 'No, I'm not doing that.' "You will not find any political party going into the next election offering to cut health spending.

"It hasn't happened in my lifetime, it won't happen next year and it won't happen whenever the next general election is.

"None of the political parties will do that. This is a scare story.

"Both governments north and south of the Border are using the private sector to do operations, mainly to hit targets.

"Over £100 million of private sector work has been done in Scotland, by the Scottish Government.

"Now the degree to which you use the private sector is a matter of real controversy but I think what they are doing is quite deliberately using the health service as a potent weapon.

"But that's what they are using it as, a weapon, and they are ignoring the fact that, according to John Swinney, never mind me, we would have real problems funding the health service and other public services."

If the Yes campaign's claims about the health service really boil down to an argument about the economy, as Mr Darling suggests, he is scathing about the SNP's proposals.

Mr Salmond has outlined a vision of increasing an independent Scotland's tax revenues by £5 billion by 2030 through a combination of borrowing to kickstart the economy, improving productivity, increasing immigration and getting more people, especially women, into work.

His flagship proposal is a 3p cut in corporation tax, the main tax on business profits, designed to attract new investment and give big firms in Scotland a competitive advantage over rivals south of the Border.

But Mr Darling says: "Whatever country we happen to end up in, all countries are emerging from one of the deepest economic downturns in a hundred years.

"If Scotland were an independent country, a new kid on the block as it would be, the idea it could run a completely different economic policy is just nonsense.

"As the Institute for Fiscal Studies has said, it would have get its debt down to an acceptable level.

"The Nationalists blithely say we'll borrow more but when the markets look at countries, especially smaller countries that borrow more, they say, could you pay it back?

"What the markets have done now is clock the fact it seems to be an article of faith among the Nats that they are ready to default on debt, If you threaten to default on debt, you pay a price for it.

"It's not just governments it's people in the country who pay a price for it. The markets will have you for lunch."

He adds: "There is not a political party in the land that says it does not want to increase productivity.

"But the productivity rates they need to get would exceed anything that's been achieved by America, France, Japan, Germany, Sweden in the past 25 years.

"Productivity is not like a tap, it's not something you can just switch on.

"To a large extent it depends on firms rather than governments.

"Frankly, cutting corporation tax by 3p more than whatever the Chancellor of the Exchequer in Downing Street does is a race to the bottom and your bigger next door neighbour could easily undercut you.

"Amazon and Starbucks paying less tax? Surely that cannot be a priority.

"The White Paper has one page of numbers in it relating to one year and they are now wildly wrong now because the oil revenues are way down on what they thought.

"They are asking people to vote for independence on a completely threadbare prospectus.

"As we get closer to polling day, let's assume the polls stay close, people should have a long hard think about this.

"This is for real. It's not some sort of experiment. It's not some sort of trial, its for real.

"If we vote to go, we take all the consequences of the lack of any credible economic policy."

"How are we going to pay for pensions. This is what people must really concentrate on. The stakes could not be higher."

At the heart of Mr Salmond's economic plan is his proposed currency union, a plan to share the pound and Bank of England under a formal agreement with the rest of the UK.

The plan has been ruled out by the Conservatives, Labour and Lib Dems, who argue it would be too restrictive for Scotland and too risky for the rest of the UK. Mr Salmond has resisted repeated calls to outline his preferred alternative, though he has hinted he would keep the pound unilaterally, without support from the Bank of England, while refusing to take on Scotland's share of the UK's national debt.

Other options, he says, include creating a new Scottish currency.

Mr Darling says the SNP is in "chaos" over the currency and has led demands for Mr Salmond to spell out his Plan B but what is his own view? What currency does he believe an independent Scotland should use?

He says: "They are all rotten options. My starting point, as somebody living in Scotland, is what would be best for us?

"Having the pound sterling works because the pound sterling is backed by the Bank of England, which is backed by the UK Government, which is backed by 60 million taxpayers.

"That's what makes the pound what it is."

Mr Darling clearly has no intention of joining Mr Salmond's "Team Scotland", the cross-party group he aims to assemble to negotiate with the UK following a Yes vote.

He was invited to do so by the First Minister during the second of their live TV debates.

"I took it in the spirit in which it was intended," laughs Mr Darling. "It's Alex Salmond at it. It's a tactic."

The Better Together organisation he leads is made up the Conservatives, whose supporters have provided much of the funding, Labour, whose activists are doing most of the campaigning on the ground, and the Lib Dems.

Mr Salmond has made much of Labour's "alliance" with the Conservatives, especially when pitching his message to traditional Labour supporters.

The nature of the three-party coalition has also appeared to constrain Mr Darling at times.

As a Labour man he believes the UK works for ordinary Scots as a vehicle for social justice, allowing resources to be pooled widely and shared more fairly.

That's not quite how his Conservative colleagues see it.

But he dismisses the idea that differences between the Better Together partners have inhibited its ability to present more positive arguments for staying in the UK.

He says: "I have no difficulty working together with people who believe we are better off as part of the UK than we are apart.

"In relation to economic and social policies, of course, I take a different view.

"I'm a Labour politician, I'm a Labour MP. I've been very critical of George Osborne's austerity approach as I have been of the austerity approach forced on Europe by Germany.

"Equally, my priorities in terms of social justice and fairness are different from the other parties.

"I don't have any problem with that and they don't have any problem with that.

"No-one has ever told me: 'You can't say that.' Far from it."

Another area where the three Better Together parties have their differences, at least when it comes to the details, is devolution.

All have promised to hand Holyrood more powers over income tax and welfare in the event of a No vote but their competing plans, which they intend to put to voters at next year's Westminster election, may prove hard to reconcile.

With senior party figures on all sides calling for consensus, might the differences between the parties see further devolution kicked into the long grass?

Mr Darling says not.

"I don't think that will happen because I don't think people have given their personal words without knowing they will pay a very high price, if they break their words.

"Look what happened to Nick Clegg with tuition fees.

"People may have a jaundiced view of politicians but, if you say something so strongly, you've then got to deliver."

He adds: "My starting point is that it makes sense to devolve as much as we can where those decisions are best taken here.

"There are some things, like the funding of pensions, like economic policy generally, where there is an argument for doing things on a UK scale.

"But if you look at the proposals made by the three non-nationalist parties I think that's a very good starting point."

Referring to the two Scotland Acts that created the Scottish Parliament and, from 2016, will give it responsibility for raising about half of the income tax paid in Scotland, he adds: "Yes there are some differences but no more really than there were prior to 1997 or 2012.

"My view is the more consensus the better when it comes to constitutions because otherwise there is a risk of chopping and changing.

"But I'll tell you this, I'd rather be inside the UK arguing the finer points than outside the UK pleading for a currency union, which is basically the choice we are facing.

"People in Scotland want change but the irony of it is that if you vote for independence you are going to get austerity-plus, you are going to find it more difficult to fund things in the future."

Scots go to the polls next week. Both sides will spend the coming days talking to undecided voters and ensuring their supporters make it to the polls.

Has Mr Darling, who might reasonably have retired into comfortable obscurity as a backbench Edinburgh MP after Labour left office, says he is glad he took the job as boss of Better Together.

But has he actually enjoyed it? He picks his words carefully.

"Two and half years is a long time to spend campaigning, I thought it was America that did that," he says.

"But you do get invigorated in the closing stages - you can see the finish line and you get energised."

As for his future, he's not yet making plans.

"I'll have a think about what I do next," he says.