As the Parliamentary ­­­ Under-Secretary of State for Scotland, David Mundell must hear a few things.

He must be privy, you'd hope, to conversations behind the scenes. The MP for Dunfriesshire, Clydesdale and Tweeddale, our lone Tory at Westminster, surely has an idea of what is going on.

When Mr Mundell declares that there will be no deal over Trident and a currency union in the event of independence, therefore, he is both right and revealing. Right because a future SNP government would have only limited room for manoeuvre over nuclear weapons, if it even wanted such a thing, and revealing because his remarks supply another piece in the jigsaw.

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When an anonymous senior coalition figuresaid last month that a currency union would "of course" be agreed, he did more than expose the posturing of George Osborne, Danny Alexander and Ed Balls. This not-so-deep throat also inspired quick improvising on the Unionist side. If all that was the case, a quid pro quo deal over Trident "had" to happen.

Mr Mundell knows better, perhaps because he knows Scotland better than London strategists who have never seen a horse they couldn't trade. As we reported yesterday, the minister recognises a "non-starter" when he encounters one. He used to believe (or hope) otherwise. Now he says the SNP could "never go back on the Trident thing. There is no deal to be done".

So what do we say to that? Those who incline towards a No vote might cling to the promises of Mr Osborne. That sometimes counts as a plan. Those who mean to vote Yes can meanwhile take satisfaction from watching a skein of disreputable argument unravelling. They can (and should) say, a currency union makes sense for all, but no one in these islands needs Trident.

You would expect no less. You will certainly get no less in the months that remain until September 18. What, though, about the people who have yet to decide how they will vote in the referendum? There are a lot of them about. They tantalise pollsters, politicians, journalists and activists alike. Scotland's future rests in their hands, it is as simple as that.

The phenomenon of the undecided puts temptation in the way of anyone who attempts comment. You can end up risking clumsy Goldilocks metaphors. What would make the political porridge just right? You can end up becoming impatient. What do these people still need to know? You can even end up - probably the biggest mistake - treating the undecided as a type, the dedicated agnostics of politics.

When all that is happening, something important gets forgotten. Where's the rule that says a person has to decide in April about a vote that won't happen until September, especially when the vote is billed as the most important in three centuries? You can see why a thoughtful person would refuse to be rushed. A life-changing decision surely requires the utmost consideration.

Those who are already committed tend, inevitably, to declare that the choice is clear. I'm one of them. I can adduce everything from the decadence of the British state to the right to self-determination to the expulsion of Trident, improved governance, unlocked potential, self-respect, an end to neo-liberalism and ever on. But that's just me: convinced.

What do you say to the person who despises nuclear weapons, but can't believe the currency union thing would work? That might be unlikely, but it's not impossible. The idea that people have yet to decide because of a few identifiable issues is just as unlikely. The only consistent refrain has been the demand for "more information". Some of that has been answered; some of it is impossible to answer. But the number of people uncertain how to vote refuses to diminish greatly.

One poll this week had them at 15% of the electorate, another at 16%. This referendum is without precedent in Scotland and without many parallels elsewhere, but those are big numbers. They represent voters who have been besieged for months and years, but who remain - at least when pollsters are asking - uncommitted. It is almost as if they have decided to remain undecided.

If so, campaigners could be frustrated for a while. The polls tell us, for one thing, that Mr Osborne's currency intervention was the opposite of decisive for the No side. Support for independence increased, in fact, but - just as important - undecided voters were unimpressed. Instead of expecting to be swayed by one argument or another, this part of the electorate appears to have postponed a decision until some private deadline. If the chosen date is September 18, there are interesting times ahead.

You would suspect, instead, that opinion will harden and clarify a month or so before the vote, when holidays are done, big public events have come and gone, and the campaigns have nothing new to add.

By then, Mr Mundell's declaration will have been given its place among all the others. A UK minister believes the SNP will never do a deal over Trident: what's that worth to the undecided?

You can extrapolate a little in the meantime from what is already known. The nuclear deterrent isn't Scotland's favourite adornment. Polling says that the biggest majority of voters want more powers for Holyrood stopping short - in contradiction to the view on Trident and foreign wars - of defence and foreign affairs. Those voters know that kind of devolution won't be forthcoming if they vote No. So does independence answer the demand?

The group classified as "Labour voters" is obviously relevant in all of this. A big increase in registration says that thousands of those voters have not been in the habit of voting. Recent election results say numbers of those who do vote have switched to the SNP. Polls show, however, that many are not yet convinced by the Yes campaign. Simultaneously, "Labour voters" ponder what the next Westminster election might bring if they vote No.

Nicola Sturgeon made her pitch to this part of the electorate at the SNP's Aberdeen conference yesterday. There was no real choice about that, nor was there anything new. Ms Sturgeon's party has been fighting for "Labour voters" for decades. Those have been won - and continue to be won - in the context of Holyrood. Where independence is concerned they are susceptible to argument, say the polls, but in many cases still biding their time.

They have that right; of course they do. Undecided voters in general deserve to be respected, in fact, for approaching the referendum at their own pace and in their own way. The campaigns won't give up their attempts to find the magic words that will win hearts and minds, but those who want Scotland to say No should have grasped one thing by now.

The attempt to terrify the undecided with prophecies of a country laid waste by independence don't work and won't work. The experiment has been attempted endlessly and stands revealed as a wholesale failure. That 15%-16% group remains indifferent to concocted horrors. They are unimpressed and unconverted.

Unionist campaigners should pause to wonder what that means. It should be on a poster: Britain isn't working.