How will Alex Salmond pay for it all?

That was a dominant theme in Scottish Conservative leader Ruth Davidson's speech to the party faithful in Manchester yesterday. In a speech that went down well with English and Scottish Conservatives, she claimed the true cost of the SNP's independence promises was £32bn.

The Scottish Government is all too aware that the credibility of its prospectus rests on the question of how it will be paid for. Partly because of that, it has long favoured the establishment of an oil fund.

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The Fiscal Commission Working Group report on how such a fund might work was published yesterday and although it sets out a persuasive rationale for such a fund in principle, it will not silence those who wonder how the SNP will fund its post-independence pledges.

Oil and gas being a finite resource, it is sensible to establish a fund so that, as Finance Secretary John Swinney stressed, it might benefit future generations. The business of government is all too frequently driven by short-term considerations. Though oil and gas will not last, the riches beneath the North Sea are proving more extensive than once thought, with existing fields being further exploited by new technologies and west of Shetland promising much. There are few in any party who would disagree that an oil fund should have been established in the UK in the 1980s.

That is not the same as saying that setting one up now would produce a bonanza, however. The UK, and Scotland as part of it, is wrestling with a deficit and ongoing budget constraints that are set to last at least seven more years. Mr Swinney did not speak yesterday in terms of black gold cascading into the Scottish Government's vaults. Instead, he suggested the Government could contribute "modest sums" to a long-term savings fund.

After all, oil revenues would have to be used to fund spending in the first years after independence, as indeed they are at present. How long might it take before comfortably high levels of spending could be maintained while also paying into an oil fund? It could be many years.

It is also highly questionable whether it makes sense to save and borrow at the same time, especially while savings rates are so poor.

Pro-independence campaigners are keen to use the comparator of Norway, which set up an oil fund 23 years ago - at a time of deficit - that now has a value of £470bn. With less oil, a different economy and challenging economic circumstances ahead, however, Scotland's prospects are impossible to predict.

Meanwhile, the emotional appeals on Scotland's future continue. A lack of fire in the pro-independence campaign has been blamed by some activists for its failure to make headway in recent months, but it was David Cameron who appeared to heed that advice yesterday, speaking to Scots direct-to-camera, saying "we want you to stay". There will be more such appeals as the vote approaches. Whichever side wins the economic debate, however, is likely to win the day come September 18 next year.