LET me take you back to July 2000, which was when Alex Salmond surprised all and sundry by resigning as leader of the SNP.

On the day he announced his decision he asked me, his putative Boswell, to pop in and see him at Holyrood. What rumours, he wanted to know, were flying about him?

I related the three juiciest ones: he was having a fling with Nicola Sturgeon; he had run up eye-watering gambling debts with a bookie in Ireland; either he or his wife, Moira, was seriously, possibly terminally, ill.

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Mr Salmond's response to each was a hearty, contemptuous laugh. His real reasons for going, he said, were two-fold. First, he had always said he would step down after 10 years in charge of the party and, secondly, he owed it to Moira to spend more time with her at their house in Strichen.

For many pundits and voters Moira Salmond is even more of an enigma than her husband. Until her appearance at the First Minister's side on recent trips to Abu Dhabi, she has remained in the shadows and beyond the reach of the media. With nothing more to go on than hearsay, various portraits of her have thus emerged, a number more fanciful than others.

To some she is the power behind the throne, a latter day Lady Macbeth, albeit one who is more likely to hand her husband secateurs than a dagger. At the other end of the spectrum she is likened to a skivvy, meekly responding to her other half's every whim, always ready to jump whenever called into action, his mother manqué.

Neither, one suspects, bears much examination. It's true that Mr Salmond is not what used to be called a New Man. Does he iron shirts or dry dishes? It's hard to imagine he does, or ever did. Like many men of his generation he sees the home as his wife's domain. As Moira once said, he spends so much time solving other people's problems he has none left to spend on theirs.

She is, of course, 17 years older than him and was his boss in the civil service. It is the former that causes eyebrows to rise, as if there were something inherently weird in such an age gap. It doesn't seem to bother either of them. Moreover, for a woman of 73, Moira is highly fashion-conscious and is as likely to be seen reading Vogue as the Scots Magazine. Were it not for her, one suspects, the First Minister would dress even more like a golfer than a parliamentarian.

She is also witty. In 2007, after Mr Salmond came in from the cold to lead the SNP to victory, she made a little speech at a select gathering. Mistakenly believing that the Nationalists would be going into coalition with the LibDems, whose then leader was Nicol Stephen, she said she regretted that a Nicol rather than a Nicola would be her husband's deputy.

By all accounts she is forthright in her views and Mr Salmond jokes that he must ration her public appearances lest she speak out of turn and lose him votes. From the beginning of their marriage they decided that she would not be actively involved in politics. It is a pact they continue to keep. "One politician is enough in the family," she insisted a couple of days ago.

This is disingenuous. Mrs Salmond may not wish to be a politician but she surely is one. Behind the scenes, away from prying lenses and microphones, she is a poised, persuasive and popular operator; a calming and charming presence when all around are tearing out their hair.

As such she is an invaluable asset to her husband, the one person to whom he must and does listen and pay heed. The polls suggest Mr Salmond has a problem convincing women voters, some of whom find him smug and bumptious. Such impressions can be difficult to shift but if his ultimate goal is to be achieved they will need to be.

Were she to be persuaded to play a more conspicuous role, Mrs Salmond could help change this. In a contest that's likely to be too close to call she could yet be the difference between "yes" and "no".