SIR Ian Wood's intervention in the referendum debate came as a surprise.

In the handful of broadcast interviews he gave, expressing his view that the SNP has been exaggerating the amount of oil an independent Scotland could rely on in decades to come, he was at pains to stress that he was a reluctant participant.

He praised Alex Salmond as an able First Minister and credited the UK Government with getting the economy back on track. He stressed he had no allegiance to any party or either campaign. But, he said, he had become deeply frustrated by the number of times he had been misquoted, by the way facts and figures had been distorted whenever the question of North Sea oil was raised. The straw that broke the camel's back, as Sir Ian himself put it, was a report last weekend by think tank N-56, which describes itself as an "apolitical business organisation" but just happens to be funded by SNP donor and Yes Scotland board member Dan MacDonald. It claimed future North Sea revenues could be six times higher than the Office for Budget Responsibility has forecast.

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Alex Salmond welcomed it. Unfortunately, Sir Ian, who was quoted freely throughout the report, considered it "quite unrealistic", "utter pie in the sky" and "completely and utterly wrong and misleading", giving N-56 the dubious honour of scoring the most spectacular own-goal of the campaign so far.

Sir Ian was not the only unexpected figure to enter the debate this week. Dr Anna Gregor, who like Sir Ian is a hugely respected professional without political affiliations, felt compelled to speak out against Yes campaign "scaremongering" on the NHS. The claim that only independence could save Scotland's devolved health service was "a total and utter lie," she said, and one that was causing "unreasonable distress and panic" for patients. She told The Herald she took "a very dim view" of colleagues using their positions of trust to play politics.

Those at the heart of the Yes campaign have been surprisingly relaxed about the bruising headlines generated by the comments of Sir Ian and Dr Gregor. At First Minister's Questions on Thursday, Mr Salmond brushed aside questions about his oil forecasts, drawing on a new statement by Aberdeen University expert Professor Alex Kemp insisting the industry's maximum estimate of 24 billion barrels (a figure that's been repeated over and over by the Scottish Government) was "plausible." Later that evening, over a quick drink at a nearby bar to celebrate Holyrood's final day of business before the vote, a group of government advisers was in good heart. "We're happy to be talking about oil," said one. "This is a question of whether there is a very large amount of oil or an extremely large amount." Energised also by the First Minister's pep talk to SNP MSPs, they were eager to get stuck into the final three weeks of campaigning. They know they are behind but they are confident about their message (especially on the NHS) and are taking heart from reports that large numbers of the so-called "missing million", folk who rarely vote and who have fallen off the parties' radar, are backing Yes.

The No campaign, however, believes the comments by Sir Ian and Dr Gregor are seriously damaging for its opponents and will seek to exploit them in the days and weeks ahead. It's not only their substantive points that have struck a chord with the No side, it's their reason for speaking out - their belief that people are being misled.

For a long time, Scots Labour leader Johann Lamont has attempted to use First Minister's Questions to cast Mr Salmond as a man who plays fast and loose with the facts. It hasn't worked, at least not according to his consistently high approval ratings. But on Monday, when he faces Alistair Darling in the final live TV debate before the vote, his whole campaign will be portrayed as untrustworthy. The messy business of Mr Salmond's non-existent legal advice on an independent Scotland's EU membership, John Swinney's over-enthusiastic comments about currency talks with the Bank of England, oil forecasts, the NHS - all will be raised by the Better Together leader. With just three weeks to go, the issue of trust is about to take centre stage.