Ruth Davidson is the leader of the Scottish Conservative and Unionist Party. She is a Member of the Scottish Parliament for Glasgow.

Nailing custard to a wall. It accurately describes how it felt last year trying to pin down Alex Salmond on the economics of independence.

Time after time, our erstwhile First Minister would endeavour to dodge, weave, bluster and bluff his way past fair questions. Projections on Scotland’s oil revenues were hopelessly optimistic – wouldn’t it be a good idea if he took actual receipts into account? Salmond would pop up on his hindquarters and trot out the usual lines about how his opponents were doing Scotland down. By contrast, his numbers were “reasonable” and “sensible”. Move along – nothing to see here.

Thankfully, the majority of Scottish voters opted for us to remain part of the UK last year. Salmond lost. But, remarkably, the SNP managed to skip free from that defeat without having to conduct a post mortem. Immediately after the referendum, the debate in Scotland moved onto a plan to deliver more devolution to the Holyrood parliament. Then the general election occupied everyone’s attention. The SNP lost the battle – but it has never been forced to examine the fundamental reasons why. Salmond and his team have never really been put on the spot.

That may now be changing. Last week, in a devastatingly honest blog, one of Salmond’s closest allies came clean. Alex Bell, the SNP Government’s head of policy in the run-up to the referendum, declared that the SNP’s model for independence was entirely bust.

It is worth quoting his comments more fully because they deserve a wider audience.

“The SNP’s model of independence is broken beyond repair,” wrote Salmond’s former number two. “The party should either build a new one or stop offering it as an alternative to Tory cuts.”

He continued: “The campaign towards the 2014 vote, and the economic information since, has kicked the old model to death. The idea that you could have a Scotland with high public spending, low taxes, a stable economy and reasonable government debt was wishful a year ago – now it is deluded.”

It was, he added, “debatable” whether a separate Scotland could maintain UK levels of spending. We must assume, he added, that leading SNP figures – including Nicola Sturgeon and John Swinney – now know that “the old model, once optimistic, is now dead”.

“Reasonable,” Salmond had told the Scottish people. “Sensible,” he assured us just a few months before we voted. Actually, no. It turns out that, in private, his team knew it was “wishful” at best, and “deluded” at worst.

So what, you might argue? After all, the vote was had, and most Scots would now dearly like to move on – and I am one of them. Many people in Scotland now want sleeping dogs to lie and for the whole row over the referendum to go away.

The trouble is it isn’t going away. The SNP is still pushing for ‘Indyref 2’. Only this week, an independence supporting newspaper was handing out free ‘Yestiny’ lapel badges for those who want to go through it all again. Car stickers with the phrase “Yes2” are de rigeur for SNP fans. If she wins next year’s Holyrood elections, Nicola Sturgeon appears ready to oblige them as soon as she thinks the SNP can get away with it. The pledge before the referendum that it would be “once in a generation” is another of the SNP’s “reasonable” arguments that has been shown up to be false.

Bell’s warning was therefore important and well-timed. And the SNP’s response to it has been both telling and utterly insufficient. Rather than engage with any of his points, a spokesman pointed to opinion polls to claim that support for independence is rising – before attacking the UK Government’s delivery of more powers for Scotland.

Leave aside the factual inaccuracy of the second point, this kind of non-response only serves to illustrate the SNP’s inability to acknowledge, still less examine, the weaknesses of its own case on independence. It’s easy to see why. Doing so would be to accept that it had pulled a colossal fraud on people in Scotland last year. And it would also put a handbrake on the SNP’s most prized possession – its own sense of momentum. The SNP might have to do the unthinkable; accept that it had got something wrong. And that cannot be allowed to happen. Salmond’s memoir of the referendum was entitled The Dream Shall Never Die. Actually, for the SNP, the truth is that the dream cannot, indeed, must not, die – if it did, then the whole edifice of an irresistible Nationalist movement surging forward would be brought down.

Or maybe they can prove me wrong. Nicola Sturgeon and John Swinney are politicians who are of a different mould to Salmond. They have pursued a less bombastic approach to independence than their predecessor. So are they up to the task of showing it? Here’s a challenge for them. In the week since Bell’s blog, we had not heard a peep from any SNP politician on the core facts of his case. Astonishingly, no-one has sought to rebut him or engage with his arguments at all. Sturgeon or Swinney should rise to the challenge. Either they should set out how an independent Scotland of high public spending, low taxes, a stable economy and reasonable government debt isn’t “wishful” or “deluded”, but “reasonable” and “sensible”. Or they should accept Bell’s points and show they have a plan which no longer relies so heavily on Salmond’s brand of belligerent and baseless assertion.

Until they do, the SNP’s attacks on this UK Government’s economic plan ring utterly hollow. Their own plans for separation have been exposed now as utterly fraudulent. They tried to sell the people of Scotland independence based on an economically dangerous lie – in full knowledge of their own deceit. Unless they can answer to that charge, why should Scots listen to them on anything else?​