Adam Ingram, the former Labour MP for East Kilbride, does not speak for the Better Together campaign.

We had better make that clear from the outset. Party stalwart and long-serving armed forces minister he may have been, but it seems Mr Ingram harbours thoughts on constitutional matters that are not supposed to be spoken.

One popped out when the erstwhile defence and security specialist was contributing to a conference on his favoured topics at Glasgow University over the weekend. For whatever reason, Mr Ingram chose to wonder about the consequences of a close referendum vote next September and even, perish the thought, about a narrow win for the Yes side.

This one is not in the Unionist script. That runs, roughly, as "Taking nothing for granted. All to play for. But, you know, look at those opinion polls, eh?" Hence the first interesting question: what persuaded Mr Ingram that Yes is even within the bounds of possibility? And if he does believe it possible, why allow the idea the oxygen of publicity?

Mr Ingram wasn't done. As we reported yesterday, he speculated about "a very tight decision". He asked his listeners: "Let's assume it's 51:49 in favour of secession, independence, separation, on a very low turn-out? Although people talk about this [referendum] as the biggest event for 300 years, it's only the beginning of an event."

What on earth was that supposed to mean? Wasn't the Edinburgh Agreement between David Cameron and Alex Salmond supposed to represent the rules of engagement? And is it not implicit in the deal that the vote will settle matters, for a few years at least? That's the Unionist line, the Downing Street line. Nationalists will not abandon their convictions, but the hope of a fresh referendum will perish, by the usual estimate, "for a generation".

Mr Ingram said something else entirely. In one sense, it was slightly shocking, unless you brought to mind George Cunningham's wrecking amendment to the 1978 Scotland Act. That was tolerated and accepted. Ingram might be the noise of a loose cannon. But when he asked why we "assume there is only going to be one referendum", he was not stretching credulity. Those who fear nationalists will never rest might have to contemplate an equal and opposite reaction to a Yes vote.

The idea is not likely to fill the average voter with joy. Most people, having listened to the arguments for months or years, want this thing settled. Even nationalists who refuse ever to give up on independence know that securing another referendum could take a very long time. Mr Ingram, in contrast, seems to see a window of opportunity in the (roughly) two-year transition period between the vote and the restoration of independence.

The slightly shocking part of his argument was in the blithe questioning of the popular will. In this corner of the Westminster world, it seems, the voice of the people doesn't carry far, not when there is the slightest opportunity to call a democratic referendum into doubt. In effect, Mr Ingram was proposing a 21st century version of Mr Cunningham's assault on legitimacy. The Yes side cannot just win, it must win by a big - if unspecified - margin.

Mr Ingram said: "Scottish members of parliament will continue after 2014. They will fight - possibly - for what they believed in before. And that would be the Union.

"The body politic itself - the people - may decide they want the politicians to reverse the decision. Do we listen to the people? Or do we listen to the parties?"

In other words, Westminster's preponderance of Unionists might scare up doubts and be minded to overturn a narrow Yes vote as insufficient. The idea might be outrageous; the possibility is not implausible.

For starters, you can easily imagine a Unionist MP declaring the rights of non-resident Scots had been violated in a 51:49 decision. In such a situation you could expect plenty of talk about legitimacy.

But how would the government of the day respond? In 2014, the coalition will still be around. By 2015, Labour could have been returned to power. In that year, as Mr Ingram points out, its ranks will contain a lot of Scottish MPs with nothing left to lose. No matter what happens, they could make a lot of noise about Britain being broken up just because a mere 51% had carried the day before "doubts" set in. Mr Cunningham manufactured his 40% threshold for precisely those sorts of reasons. And it worked a treat.

In the upside-down world of British constitutional doctrine, Westminster is perfectly "entitled" to overturn a referendum. The sovereignty of the people, history's comfort for many Scots, cuts no ice down by the Thames. The tradition is not and has never been accepted, despite all those Labour signatures on the Claim of Right. Nothing compels the Commons to accept a Yes vote.

Mr Ingram's devilish little argument falters a little over turn-out. For his perfect storm to gather force, the now-usual apathy would need to prevail to shape "the beginning of an event". There is very little sign of that happening. Thus far, the numbers of those reporting themselves as "certain to vote" are easily better than respectable. The point, perhaps, is that in this species of Labour thinking September 2014 need not be definitive. MPs and parties need not defer to the people's expressed will.

Winning a vote need not be sufficient. If nationalists refuse to accept defeat, why should Unionists? So the arguments would divide still another generation as a service brought to you by people who otherwise dismiss constitutional politics as a distraction.

Mr Ingram might well have been talking from the top of his head, a lone, speculating voice, a figure oblivious to the hell that would be raised by the overturning of a Yes vote. He makes it clear, nevertheless, that those who regard independence as secession, all treaties forgotten, have plenty of tricks left in their sleeves. The two years between 2014 and 2016 would allow many opportunities for mischief.

This argument has been a long haul for the Scottish people and it is not over yet. The controversy over naval dockyards has been proof we are a long way from the finishing line. For all that, voters are entitled to expect a referendum amounts to a decision. Quibbling and tinkering - cheating, indeed - after the fact will not sit well. Mr Ingram speaks like a Westminster parliamentarian when he speculates about such manoeuvres.

The fascinating detail is that the entire edifice of his argument rests on a Yes vote. The former armed forces minister did more than just admit the possibility. The pity is he did not explain that part of his thinking fully at the Glasgow security conference. If nothing else, he seemed not to take all those opinion polls at face value.

The game is far from over, even as some of the spectators grow weary. That misalignment is inevitable, but a pity. For my part, I won't begin to think too seriously about the polls until next spring. We should thank Adam Ingram meantime for an invaluable glimpse of the Labour mind.