IN research for my book on Labour’s election campaign – Five Million Conversations: How Labour Lost An Election And Regained Its Roots – I gained access to some of the party’s extensive private polling in Scotland. It points to even deeper difficulties for Labour than are immediately obvious.

First – and briefly – the good news. After the so-called "challengers debate" in April when Ed Miliband took on Nicola Sturgeon minus the presence of the Prime Minister, the SNP's own research suggested that their party leader's line – that there wasn't "enough of a difference" between Labour and the Conservatives – was particularly potent. It garnered them support from some of those who had voted No in the referendum. With Cameron now facing Corbyn at the Westminster dispatch box, this is palpably no longer the case – if it ever was.

But while that attack might now be blunted, Labour's own focus groups and its wider polling suggest that the adoption of an anti-austerity agenda by Corbyn won't in itself transform his party's prospects.

In fact, the research delivered the party a double whammy. Pow! Labour were seen to be promising more than the country could afford.

Wallop!! the SNP were expected to spend even more – but, crucially, to spend it more wisely.

So if Labour were simply to argue for more spending and less austerity, that could be seen as irresponsible of them – while the same message delivered from the mouth of Nicola Sturgeon would be more credible.

But the news grows grimmer still for Labour. In the words of one insider, their focus groups "didn't tell us whether we were f***ed or how we were f***ed – just the extent to which we were f***ed".

If a week is a long time in politics, then 16 years is an absolute aeon. In 1999, just ahead of the first elections to the Scottish Parliament, I wrote a feature for this newspaper examining how Labour had overturned an SNP lead in the polls, and were on course for victory. The strongest weapon in their armoury? A slogan that played on people’s concerns: "Divorce is an expensive business." At their focus groups, Labour had – crudely –found that male voters by and large didn’t like the expense that could come with setting up an independent country. Women voters, generally, didn’t like the idea of separation. At the time I called it "negative and effective".

This paper reported a similar theme before last year’s referendum: that senior figures in the No camp were describing their strategy for success as Project Fear.

But examining the private polling for my book, the "fear factor" weapon might have to be decommissioned, as it now appears obsolete.

At the election, Labour campaigned hard against the SNP policy of Full Fiscal Autonomy – so robustly in fact, their tactics were described as Full Fiscal Assault. It was thought – in the words of one Labour strategist – that this could be a "worry point" for former Labour voters.

But it was Labour that needed to worry – the party’s focus groups delivered a shock.

Target voters were resisting the fiscal assault. Partly, there was a "cry wolf" element to it – No voters had bought into the risks of independence in September and some were weary and just weren’t keen to see the tactics of that campaign replayed.

But far more devastating was this finding: some of the key voters – "soft" SNP and ex-Labour – now expected to pay more for greater autonomy, and apparently accepted this. As one focus group member put it: "Och, divorce costs money doesn’t it?"

So no longer just expensive business, but possibly a price worth paying.

An internal memo said voters did see Labour's central charge – that the SNP would have to raise taxes or cut spending – as "plausible". But it added – and here's the rub – this wasn't "particularly scary".

So the expense of independence no longer engenders the fear it once did.

Jeremy Corbyn is probably the leader best placed to adopt a change of tack. But he will have to tread carefully. On his visit on Thursday, he made it clear Kez Dugdale is in charge in Scotland. Nonetheless, will his now monthly visits undermine that impression? The most resonant "negative" of all in Labour's private polling was that the party would "put Westminster before Scotland".

At UK level, senior Shadow Cabinet members are beginning to discuss how they handle the SNP and whether completely distancing themselves is sustainable.

And in Scotland, there are calls from some for political independence – not for the country, but for the party – with Scottish Labour becoming a sister organisation to the UK body. The former first minister Henry McLeish will be saying more about this shortly.

At a packed rally on the fringes of Labour's conference in Brighton last week, Kezia Dugdale was cheered to the rafters by the party's modernising wing when she declared: "Nothing is inevitable." But it appears that a new approach from Labour is a near certainty. That's because the party’s own polling suggests it isn’t just desirable, but necessary if a further erosion of support is to be avoided.

Iain Watson is the BBC's Political Correspondent, his book Five Million Conversations is published on October 10 by Luath Press