FOR all the talk of possible triggers for a second independence referendum - the UK's withdrawal from the EU, the renewal of Trident, disappointment over Holyrood's new powers - there is only one that counts. Only if the polls show a substantial and sustained lead for Yes will Nicola Sturgeon call a re-run of last year's vote.

The other triggers, the "material changes in circumstance," as the First Minister puts it, are merely scenarios that might deliver the required surge in support.

Who knows if that will happen over the next five years? One thing is sure, though: the SNP will be prepared. As Ms Sturgeon told Holyrood journalists during a lunch in Edinburgh this week, her party is thinking deeply about last year's defeat, about "what worked and what perhaps we didn't do as effectively".

There is a growing acceptance among senior Nationalists that the economic case for independence will have to be overhauled. The question of currency remains fraught. "That's always going to be a difficult issue," Ms Sturgeon admitted, though she said she wasn't losing sleep over the issue. The state of Scotland's public finances will also have to be addressed. Promises of a more generous welfare system, better pensions and universal free childcare always looked optimistic, to say the least, when oil was $113 dollars per barrel. Now it's less than half that.

Serious Nationalist thinkers have come to the conclusion - wisely, I'm sure - that they will have to be more candid with voters if there is a next time.

Ewan Crawford, a former Scottish Government adviser, has written in The Herald of the need to emphasise the economic potential of an independent Scotland in any future campaign; look to the longer term rather than the starting point.

It is a message borne of necessity. If the SNP is to fight another campaign in the foreseeable future it will have to overcome the awkward fact that an independent Scotland would begin life worse off than if it remained in the UK. Moreover, dwindling oil revenues mean the gap would be wider in 2020 than it would have been last year.

That presents a formidable challenge even to party as dominant as the SNP.

Some thinking, however, is emerging and it goes like this. If a window of opportunity is to present itself, it will come in 2020 or early 2021, after the next UK election. A second Conservative government would prove the final straw for the so-called "reluctant No's". In addition, the SNP could by then argue that Scotland's finances were in better shape than the UK's were when Scots rejected independence the first time around. True, an independent Scotland would be running a deficit but nothing worse than the UK in 2014 and manageable by international standards.

The figures bear this out. The UK's deficit, as a proportion of national income, was five per cent in 2014/15. Scotland's, by 2019/20, is projected to be 4.6 per cent.

The Nationalists will do their best to draw a veil over the UK's finances by the same date. Britain as a whole is projected to be in surplus by 2019/20. The difference between Scotland's and the UK's fiscal balance - 4.9 per cent - is equivalent to £9.7billion, according to the Institute for Fiscal Studies' most recent forecast.

Ms Sturgeon believes Scots who voted Yes last year will support independence again "in all circumstances".

If support really is so solid, a future Yes campaign would only have to persuade a limited number of Scots that an extra £10billion or so of borrowing, tax rises or spending cuts was preferable to having George Osborne as prime minister. They might well feel confident of achieving that.

There is a huge irony in all this, of course.

The main reason Scotland's finances are improving (Scotland's deficit as a proportion of GDP was 8.6 per cent in 2014/5) is George Osborne's austerity programme, the policy the SNP opposes so vigorously.

Neither the SNP nor Labour, whose pre-election plans to reduce the deficit were all but identical, would have returned Britain to surplus by 2020 or have reduced Scotland's deficit so sharply.

It shows, if nothing else, how well the SNP's aims were served by David Cameron's re-election in May and would be again by another Tory victory in 2020.