AMID all the talk about the if, when, how and why of a second independence referendum, a technicality is being overlooked: David Cameron says you can’t have one.

In fact, the obstacle is a bit more than a technicality. Cast your mind back to the Edinburgh Agreement signed in October 2012 by Cameron, Alex Salmond, Michael Moore and Nicola Sturgeon. The deal struck was intended to make a referendum acceptable to all and, just as important, to make it legal. With the mighty edifice of the British constitution at stake, that meant Westminster legal.

Such was the path the SNP had chosen. It remains the reason why, even with a Holyrood majority and as many Commons seats as anyone can decently possess, the party doesn’t simply declare independence. Salmond wanted to avoid courtroom challenges, boycotts of the vote, and worse dissent if or when Yes carried the day. Automatic international recognition, among other things, was at issue.

Those prizes were not without a price. One was having to endure Westminster behaving as though a referendum was a gift bestowed rather than the recognition of a democratic right. Nevertheless, anyone who thinks the Edinburgh deal was a bad idea should consider Madrid’s behaviour towards the Catalan independence movement. The dull legal stuff can have messy consequences.

The fact remains that Scotland’s referendum, the great carnival of popular democracy, was steeped in irony. The votes of 3.62 million people were permitted – the word is accurate – thanks only to an Order in Council approved on February 12, 2013. “The Queen’s Most Excellent Majesty in Council” gave the nod.

It mattered then and it matters now. In the here and now, Cameron’s most recent word is that he’s in no mood, as he probably puts it, to go through all that again. Confronted with the question in July after Salmond had appeared on the BBC and mentioned austerity, “Brexit” and broken promises as “moving things towards a second referendum”, Cameron was dismissive.

“I think it is important that a referendum is legal and properly constituted,” he said, “and that is what we had, and it was decisive, so I do not see the need for another one.” After the usual clarifications, this turned out to mean he would not agree to another vote during this parliament. That remains the situation. The Queen’s Most Excellent Majesty can relax. And the SNP, believes the Prime Minister, can think again.

You can’t entirely blame Cameron. Why offer a rematch when the last win was a lot less comfortable than anticipated? Why on earth agree to go back into the ring when your opponent looks so much stronger than before?

Sturgeon, as we saw again last week, is not going to settle for that. She doesn’t want a second referendum if she is not guaranteed to win, therefore she wants to call the thing at the most propitious moment. For that to work, she must first secure the right to call the referendum.

She too is under some pressure, after all. Many on the Yes side will only stop nagging her if a vote is promised soon, ideally for a week on Thursday. Equally, this might be as good as it gets in terms of popularity for the SNP and for Sturgeon. She has to be ready.

We have been here before, of course, at least in constitutional terms. The difference this time is that Cameron has an entirely well-founded fear of losing. So Sturgeon has begun to lob a few shells at his trenches. One shell went off – metaphorically, but you can’t have everything – in a speech by the First Minister to the Scottish Parliamentary Journalists’ Association in Edinburgh.

Labour’s Kezia Dugdale and Willie Rennie of the Liberal Democrats have both said recently that they would grant the equivalent of a free vote to their parties in the event of a second referendum. Last week, Sturgeon called this “curious”, reasonably enough, given that a she says, the pair also believe such a vote should be ruled out “forever and a day”. It is an odd sort of liberty that can never be exercised.

Sturgeon in effect offered a deal: she wouldn’t push for another referendum unless there was a big movement in public opinion, or a “material change in circumstances”. But if either were to happen, shouldn’t Dugdale and Rennie “agree it would be wrong” to rule out another vote “indefinitely”? How, in common sense, could they do otherwise?

It was a clever move, mostly because it was perfectly logical. The fact that the three parties will never agree on a definition of “material change” is a bit of a problem, however. Equally, the scenario mentioned repeatedly by Sturgeon, the one in which Scotland is dragged out of the EU against its will by English votes, these days looks like an outside bet. What else would pass muster and allow common cause over the right to a referendum? One clever move doesn’t get us far.

It begins to pin down Labour and the Lib Dems, however, at a moment when they are bruised, battered and vulnerable. Voters might or might not be eager for a second referendum. Telling them they can’t have one is a different matter. Decreeing that people will be denied a say even if circumstances change, even if there is a groundswell of demand, and even if Labour and Lib Dem politicians want to “examine the independence case”, is daft.

Besides, the second referendum will be a different affair from the first. There will be no Better Together, for one thing. Even before Joe Pike published Project Fear, his mesmerising account of a shambolic and comical campaign, it was plain that Labour in Scotland would not touch such an enterprise again, bargepole or no bargepole. The party saved the Union, after a fashion, and all but destroyed itself. The Lib Dems simply compounded existing misery.

Dugdale and Rennie don’t need to be reminded. They probably react to the word “referendum” by pulling the duvets over their heads. Once they emerge blinking into the light, however, and once Cameron (or whoever) is forced to deal with reality, the Labour and LibDem leaders might even see advantages in a second campaign. If individuals are free to pursue the arguments for and against, if they are not shackled to the cynical compromise of Better Together, all parties – and all voters – might benefit.

Dugdale need only ponder the tens of thousands of former Labour voters who switched to the SNP before and after – especially after – the last referendum. Like Jeremy Corbyn, she might be wedded personally to the Union. That wasn’t true of her party in Scotland last September and it will not be true next time around. Why risk carnage all over again? If next year’s Scottish elections produce the expected result, Sturgeon’s challenge will become inescapable.

Not every voter is partisan. Not every voter reached a settled opinion years ago. Not every voter wants to be conscripted to one side or another. Genuine argument in a genuine debate that allows individuals to reach their own conclusions is a prize in its own right, and a prize worth having.