Separation means once you go, you go. You can't come back

In many politicians this might seem like playing hard to get but Alistair Darling is not someone given to social affectations. Candour not coyness defines him. Yet why is this proud Scot, former Chancellor of the Exchequer and committed fiscal Unionist, so reluctant to spearhead a campaign against the man who would sever Scotland from the United Kingdom?

So far Alex Salmond has steered the independence argument exactly to his liking. Meanwhile those who disagree with the First Minister's plan for radical amputation are without a central figure whose gravitas could pull together a robust opposition. "Look, this campaign has to be run in Scotland," says Darling, whose Edinburgh South West majority for Westminster increased in 2010, bucking the General Election's trend of humiliating Labour defeats.

Now, in moving from Downing Street to the back benches, he appears a decade younger, as if the clenched posture of high office had been ironed out of him. "Certainly I intend to play a full part in making the case for the UK because it's something I care about passionately. The financial risks of separation, especially at this time of global uncertainty, are immense."

It is not that Darling believes Scotland incapable of going it alone but the country, he says, needs to revive that early entrepreneurial vigour which gained distinction around the world, and it is better poised to achieve that by remaining within the UK.

"Scotland has much to contribute but when the centre of economic gravity is moving remorselessly from the west to the east to the south, the big question is how we best equip ourselves to build and develop new industries to meet that challenge. When we come to the referendum, it's not like electing a government which if it turns out to be no good you then put in another lot. Separation means once you go, you go. You can't come back."

That prospect requires clear-sighted vision: "If you ask me what's the one thing that needs to be put right in the Scottish Parliament, it's the huge imbalance of Scotland's ability to spend money without having to endure the pain and political flak of raising it."

And could an independent Scotland really expect the Bank of England to act as its lender of last resort? The man at the Treasury in 2008, when the world's banks went into freefall, dismisses the notion as an absurdity undermining independence itself.

"It's not up to the bank to decide whether it would be the lender of last resort to Scottish banks. It's up to the UK Government. When we had all the trouble with RBS and HBOS, the Bank of England had to put nearly £60billion into the system to stop it collapsing. The only way it could do that was because the governor phoned me up and said: 'I take it you're standing behind this?'"

Darling offers the stark reminder that at that time the combined balance sheet of RBS and HBOS was 30 times that of Scotland's GDP. "Now, if Scotland had said: 'I'm guaranteeing these banks,' people would have replied: 'oh, no you're not. The Scottish population isn't big enough; you don't have the money to do that.' So, that's where size matters."

This reference to Black October brings conversation round to Darling's memoir, Back From The Brink, a frank appraisal of New Labour's remarkable journey from boom to bust which he will discuss at Glasgow's Aye Write! book festival on Saturday.

Darling, in whose quiet drollery there sometimes lurks an acid drop, describes his character as thrawn with that pesky Scottish talent for spotting a flaw, "the dark side no matter how bright the day". But thrawnness held good in those weeks when global stability was disintegrating and political enemies were out to gun him down.

Did Darling ever feel an impulse to cut and run? "No. There's no point closing your eyes and saying: 'please take me away' because no-one is going to do that. The one time I did feel a chill going down my back was when Tom McKillop (former RBS chairman ), and I spoke as the bank's shares were collapsing. I asked: 'how long can you last?', assuming he'd say a couple of days. And he replied: 'well, maybe two or three hours.'" In that sickening moment the Northern Rock catastrophe replayed in Darling's mind.

"Actually Northern Rock was a well disguised blessing because two things came from it. One: I saw what could happen when you get a mini-panic. The other: whatever your response, it must be far more and far faster than people are expecting. This is where the Eurozone has failed woefully."

Neither Gordon Brown nor Darling had a scintilla of doubt, he says, about what had to be done to manage the impact of what Darling calls the complete and utter car crash of the biggest bank in the world. Hence, that contentious word bail-out entered everyday vocabulary.

Darling doesn't doubt Alex Salmond is an exceptional politician but advises caution against hyping the First Minister as infallible "This is the man who wrote to RBS saying he had looked at the deal with ABN Amro and was confident it was good for Scotland. Well, if he honestly thought it was good for Scotland, he's a complete fool because it brought the entire RBS edifice crashing down. So, he does get judgment calls wrong."

Weeks before the financial world careered into near-ruin, Downing Street encouraged a vituperative attack on Darling over a Guardian interview in which he warned – from his holiday home on Lewis – that the world portents were bad. "I wanted to prepare people that this wouldn't be a traditional downturn but something much more profound."

As a result, certain colleagues, the Tories and much of the media branded him as irresponsible. "In fact the only thing I'd got wrong was that this wasn't the worst crisis for 60 years but probably the worst we'd ever had."

But how did Darling manage to work with Brown after such a vicious assault? "We've known each other a long time and although there was all this unpleasantness in the background, we were at one on what had to be done. Gordon in full flight is very impressive and he deserves immense credit for getting that agreement (from the G20 the following April). He put the officials out (of the room), shut the door and said: 'unless we emerge from here with something the markets will find compelling, we're all stuffed.".

Darling thinks that if the Eurozone had recently shown the same determination and decisiveness, maybe Britain, like America, would now be inching towards recovery.

Talk of leadership brings us back to the Scottish referendum and whether the Unionist campaign is losing ground precisely because it lacks of a cohesive, cross-party figure at the helm. "It will be a different story when we get to the shorter campaign." At that point Darling intends to spend much more time in Scotland but he's exasperated the First Minister has set D-Day so far off. "Not until 2014- that's practically the same length as an American presidential campaign."

Alex Salmond, he says, is "shamelessly" using the entire arm of the Scottish state every single day towards one goal: independence, a word which means whatever he wants it to mean. "I'm optimistic we can win because we have the right arguments. But I simply cannot abandon the House of Commons for two and a half years."

What did Darling make of David Cameron's reluctance in Edinburgh to spell out the benefits to Scotland of a No win? "To be honest I don't know what he has in mind but there's no point spending months and years developing a policy for enhanced devolution – be it devo max or devo plus – until people have decided where they're going. So, let's get the first question over and done with.

"Are we staying or are we not? I'm totally cynical about Mr Salmond's motives here. He wants the middle option on the table to muddy the waters so that whatever the result, he'll claim he's won."

As budget day approaches Darling finds himself on the other side of the Chancellor's red box. Does he miss stepping out on that famous doorstep? "I don't miss the trappings and I certainly don't miss the press calls, or coming home to do papers."

Now he and his wife Maggie Vaughan, a former Herald columnist, have time for the movies. Last film seen, The Iron Lady. Verdict: Meryl Streep brilliant, politics, shallow. And when they make the movie of Gordon Brown, who should play Alistair Darling?

"Oh, the only grey-haired actor I can think of is George Clooney, but by then he'll be so old he couldn't possibly play me."

There he goes again, looking for the flaw.

Alistair Darling will be talking about Back From The Brink (Atlantic Books) at Aye Write! in the Mitchell Library, Glasgow on March 17 at 3.30pm.