AS rival politicians in Labour both north and south of the jostle for power, the SNP's leadership looks secure for years to come.

In large part, that is due to the popularity of Nicola Sturgeon, long seen the natural successor to Alex Salmond who she served as deputy for a decade. Her performance in the general election campaign, which saw her rise to prominence on the UK-stage, has cemented her status at the top of the SNP, all but silencing voices that claimed her predecessor would still be pulling the strings.

However, a close-knit circle of influential figures at the top of the party, including two of the country's most prominent power couples, is another factor in allowing the leadership to control the party.

First Minister Nicola Sturgeon and her husband Peter Murrell, the SNP's chief executive, are joined in the party's inner sanctum by health secretary, Shona Robison and her husband, depute leader Stewart Hosie.

Mr Hosie, MP for Dundee East since 2005, won the battle to become depute leader in November. Meanwhile, Ms Robison, who won plaudits as minister for the successful Commonwealth Games, was promoted to health secretary by her friend and ally Nicola Sturgeon in her first cabinet after becoming First Minister.

Mr Murrell, who is seen as the driving force behind transforming the SNP from what was seen largely as a fringe party into one of the most fearsome political machines in Europe, married Ms Sturgeon in the summer of 2010, after living together for seven years.

Herald columnist David Torrance, author of biography Nicola Sturgeon: A Political Life, said: "They've all known each other a long time - Hosie first met Nicola during the 1988 Govan by election, and Shona was at Glasgow university with Nicola.

"That degree of familiarity makes the party leadership very cohesive, with little of the rivalry that lets down other parties. It means if there's clear guidance from the leader then it'll get done."

After she became First Minister, Ms Sturgeon was forced to fend off accusations that she and her husband would hold too much power, with some SNP members uneasy at the prospect.

In response, she said she was comfortable with being leader while Mr Murrell is the party's chief executive, stressing that most of the administrative oversight of the SNP is conducted by business convener Derek Mackay, rather than her husband.

Mr Mackay would deal with complaints about her, she said, although the SNP constitution states that the leader appoints the business convener each year, putting his fate in Ms Sturgeon's hands. That control has increased since then, with a resolution passed at the recent SNP conference forbidding MPs from public dissent from the party line.

However, despite some dissenting voices, a senior SNP source insisted that the close relationships at the top of the party were a positive and had contributed to the party's breakthroughs at the Scottish Parliament and now, Westminster.

The insider added: "Part of the reason for SNP success is that we have genuinely never taken anything for granted - none of us would believe the Holyrood overall majority in 2011 until it had actually happened, and it was the same last week when the exit poll came out.

"And another part of it is very good personal relations at the top, covering key people in the Scottish Government, Holyrood, Westminster, and party HQ.

"The fact that Nicola and Peter, and Shona and Stewart, are couples obviously helps in that regard - each of them is vital to what has been achieved - but I don't think it would be significantly different if that wasn't the case. It's a team effort all round."