It is a familiar refrain among divorcing couples that there were ‘three of us’ in the marriage.

Yet, for 15 years, Peter Murrell and Nicola Sturgeon that third party was welcomed in with open arms – given that it was the political party they came to dominate as its great power couple.

They surrendered to its every whim as politics became their life and their one great, shared love.

But unknown to Ms Sturgeon, apparently, another temptation was leading her husband astray, something he found entirely unable to resist, even though it would cost him his precious political legacy, his marriage and, ultimately, his liberty.

Almost from the moment they exchanged vows, and certainly within a month of that date in July, 2010, Murrell had begun cheating on everything he held dear - with the company credit card.

Over the course of the next 12 years, as court papers reveal in shocking detail, he syphoned off hundreds of thousands of pounds from party finances to fund the lavish lifestyle to which he felt a man of his standing should be entitled.

And while he and Ms Sturgeon seized the reins of power – she as First Minister, he as SNP chief executive – Murrell’s secret spending spree at the party’s expense spiralled out of control.

There were luxury leather goods, smart Church & Co shoes, Mont Blanc pens, even make-up by Estee Lauder and Jo Malone cosmetics. There was bespoke furniture and designer lighting, posh cookware and luxury watches – all the trappings of a high-achieving, high-flying executive.

Except that, for all the success he had helped his party to, it could never have afforded the salary he needed to pay for it all. So he helped himself.

His costliest purchases included a Jaguar iPace, on which he spent £57,500 of party cash, and the £124,550 spent on a Niesmann and Bischoff Smove motorhome - later to become the butt of so many anti-SNP jokes - which he stored at his mother’s home in Fife.

And then, for this once golden couple, it all fell apart. First, their grip on political power loosened in 2023 just as police launched a major investigation, Operation Branchform, into the disappearance of huge sums of money from party coffers.

Then, in short order, their marriage collapsed, apparently to no-one’s great surprise.

On Monday, the latest ignominious chapter in their fall from grace played out as Murrell appeared in court and admitted embezzling more than £400,000 from the party he and his wife once controlled with an iron grip.

At the High Court in Edinburgh, judge Lord Young described his actions as ‘a gross breach of trust’, before Murrell, 61, was remanded in custody and led away in handcuffs.

His now-estranged wife later stated she had ‘no knowledge or suspicion whatsoever’ about her husband’s criminal behaviour.

Ms Sturgeon, 55, who was also arrested as part of Operation Branchform and later cleared of any involvement, said she had been left ‘angry, hurt, sad and very distressed’ but insisted: ‘These are not my crimes. I was misled just as others were.’

Certainly, there is much to be misled over. Two Bremont World Timer Alt 1 watches, each worth almost £5,000; a £4,000 Starwalker World Time fountain pen; a £3,500 Hamilton and Inches silver wine coaster, and a Jura Giga 5 Cromo coffee machine for £3,231.90. In January 2020, he even spent £2,618 on Lalique salt and pepper grinders.

Such a betrayal seems even more remarkable given how the pair worked hand in glove for more than a decade, masterminding a string of election victories for the Nationalists since the 2014 Independence referendum.

The wound must run deep given they first met in 1988 - almost four decades ago at an SNP youth weekend which Murrell helped organise.

Back then, the Edinburgh-born party worker was ensconced in Alex Salmond’s Banff and Buchan constituency office in Peterhead while Ms Sturgeon, fresh from school in Dreghorn, Ayrshire, was a teenage firebrand.

Little about this geeky, party backroom boy - six years her senior - appeared to push any romantic buttons for her.

At youth camp and, thereafter, at intermittent encounters at party conferences, she remembered him as Mr Gadget Man. ‘He wore a belt with all his gizmos on it, including a very early Psion Organiser,’ Ms Sturgeon later recalled. ‘I was transfixed. How can anybody walk about with that attached to his belt?’

His nerdish persona earned him the nickname Penfold in party circles, after the bookish, bespectacled sidekick of cartoon character Danger Mouse.

For his part, Murrell remembered a teenage Nicola appearing on a Grampian Television programme: ‘She must have been about 18. I remember being impressed by her political skills, even at that early stage.

‘The funny thing is, I’d probably organised it… but I don’t remember recommending her.’

And yet, by 2003 the pair were an item. Ms Sturgeon had single-mindedly pursued Nationalist politics since gaining her law degree from the University of Glasgow and would end up married to the job.

At the 2003 Holyrood election campaign, Murrell was the party’s chief executive and was in daily contact with Ms Sturgeon during her bid to be re-elected as an MSP.

After the election, the need to be in constant touch was no longer there, but Ms Sturgeon recalled: ‘That’s when I thought, well, I want to be in constant communication. It was the point we both realised there might be something else.’

Both ended relationships with partners and they embarked on a relationship which, for a time, they were intent on keeping secret. Murrell, it turns out, is good at keeping secrets.

It became common knowledge after the SNP spring conference of 2004 to the concern of some in the party who felt it blurred the lines between party affairs and the affairs of state. ‘It’s like a cabal at the top of the party,’ complained one source bitterly.

Neither Ms Sturgeon nor Murrell saw it as a problem and he remained in post and she moved upwards, first to deputy and later to leader and First Minister.

If Mr Salmond, who died suddenly in 2024, was uncomfortable with the arrangement as leader and later First Minister, he did not say it publicly. He would have plenty to say about it once his protégée succeeded him and he quit the party following accusations of sexual assault for which he stood trial and was acquitted.

In his view, Murrell should have stood down as SNP chief executive as soon as his wife became First Minister in 2014.

The pair had married four years earlier in a quiet ceremony at Glasgow’s Oran Mor, days before her 40th birthday.

By then, they had purchased a £228,000 detached home on a smart suburban estate in Glasgow’s eastern outskirts.

Within months of the wedding, Ms Sturgeon was pregnant, but later miscarried - a private heartache in her marriage that was not revealed until well into her time as First Minister.

What was also not revealed until quite recently is that just weeks after they married, her husband had started embezzling funds raised from branch party tombolas and coffee mornings.

Outwardly, the couple became the model of a middle-aged, workaholic couple with no children for whom nights out on the town were never their thing, preferring Indian takeaways and TV at home.

‘The fact that we are on first name terms with the person who delivers it on Friday night shouldn’t allow you to read anything into my lifestyle,’ joked Ms Sturgeon to Good Morning Britain presenter Susanna Reid during a now-infamous ‘at home’ interview.

They hoped that inviting the cameras in would show the nation how normal power couples live, but it just highlighted how odd their marriage seemed. As she wandered round the four-bedroom property, Ms Reid was struck by the near-clinical levels of cleanliness. It was, she opined, ‘immaculate - like a show home’. Only the very expensive £1,400 Miele coffee machine in the kitchen looked like it got any regular use.

The First Minister let slip that her only domestic chore was ironing her husband’s shirts on a Sunday night ‘because, in my head, that’s me, I have pulled my weight in the house’.

But clearly, their home life was just another setting for their work life. Ms Sturgeon, who succeeded Mr Salmond as First Minister in 2014, once recalled waking up at around 3am to find her husband sitting up in bed and checking the latest SNP membership figures which had surged in the days following that year’s independence referendum.

It was, of course, the plummeting membership figures and Murrell’s role in putting out misleading information about them which resulted in him quitting his role in disgrace in 2023.

But collateral damage from the fact that the party’s leader and chief executive shared a bedroom had been building up for years prior to that point.

As the heat was turned up on Mr Salmond over sexual harassment claims, he arrived at his successor’s home in 2018 to discuss his predicament.

Murrell was in the house when Mr Salmond admitted to his wife that he had indeed behaved improperly towards a female worker in Bute House and had apologised for it. The news devastated her.

And yet, at a rare public appearance before the Salmond inquiry in December 2020, Murrell said he did not know what the meeting was about.

He said: ‘When you are married to the First Minister, who is privy to lots of information, when she says she can’t talk about something you don’t continue to say “ah, but”. It just doesn’t happen.’

Strangely, Ms Sturgeon painted a different picture of working so closely in the same line of business, saying: ‘The downside is that you just end up talking about it all the time and you never leave it outside.’

Of course, when the public pictures their marital home, it is with a blue police forensics tent dominating the front garden as officers arrived early one April morning in 2023 to unpick the clues to Murrell’s criminality.

That came just three weeks after Murrell relinquished his role as the SNP chief executive. The previous month, Ms Sturgeon had left office as First Minister.

And having split from office, the pair then split from each other. Ms Sturgeon, who stepped away from Holyrood at this month’s elections, now prefers to post pictures of herself on Instagram partying with a close group of female friends, including the crime writer Val McDermid.

Last year, she released a memoir, Frankly, fuelling speculation about her sexuality when she stated that she did not consider it to be ‘binary’. She recently said her separation from Murrell was down to more than ‘my husband being accused of crime and stuff’. But it can’t have helped.

She was, she said, focused on ‘building a new phase of life’, adding: ‘I will be making no further comment.’

For the taciturn Peter Murrell, too, a new phase is beginning. There will some adjustments though. No more designer saltshakers, coffee machines, or fountain pens.

Just prison-issue sweatpants and plastic cutlery.

It is quite a fall from the high life.