IN February, Pete Wishart, the SNP MP for Perth and North Perthshire, made a speech in the House of Commons that burned with righteous indignation.

The subject was MPs' second jobs. He touched on allegations levelled at Jack Straw and Sir Malcolm Rifkind, who days earlier had fallen prey to a sting when they discussed the possibility of doing consultancy work with reporters posing as Chinese business people.

Both were subsequently cleared of breaking Commons rules but Mr Wishart was not afraid to jump to conclusions.

"Here we go again," he raged. "Just when we think that the Westminster establishment could not be held in lower esteem by the public, something comes along to disabuse us of that notion."

Behind it all, he said, was "the possibility of cash".

The "Westminster Parliament" - he picked those words carefully - was seen as "little more than a self-serving institution for its overpaid Members".

In Mr Wishart's eyes, the SNP shone like a beacon of purity in this black hole of money grubbing rogues.

"No SNP Member has a second job, a directorship or a place on a company," he claimed.

"Our responsibilities here are our sole concern and our only responsibility. SNP Members serve our constituents and ensure that the agenda for the nation is progressed. That is what we do when we come here."

Westminster was rotten to the core compared with Holyrood, he argued, before winding up with a flourish: "There should be no second jobs, no paid directorships, no outside interests with a financial return.

"Let us work for our constituents and make them our only priority."

His speech neatly encapsulated a favourite SNP narrative.

Westminster is a failed institution, the story goes, but the SNP is different, holding itself to higher standards than the other parties. It has served the Nationalists extremely well.

This is why the Michelle Thomson affair is causing Nicola Sturgeon so much embarrassment.

It threatens to shatter the SNP's carefully constructed image as a "new politics" alternative to the rotten "old politics" of discredited Westminster.

Ms Thomson resigned the party whip, triggering her suspension from the SNP, after police launched an investigation into a series of property deals involving her company.

It is understood her solicitor, Christopher Hales, who has already been struck off, faces questions over possible mortgage fraud.

The newly elected Edinburgh West MP insists she has always acted within the law and has offered to co-operate with police in order to clear her name and return to frontline politics as the SNP's business spokeswoman in the Commons.

But whether the SNP will want her back is another question.

Whatever the outcome of the police inquiry, Ms Thomson will continue to face fierce criticism over her business practices, which involved buying homes at knockdown prices from people in financial difficulty and immediately selling them on a large profit.

"Profiteering from vulnerable families," was how Kezia Dugdale, the Scottish Labour leader, put it during First Minister's Questions.

In the same session, Scots Tory leader Ruth Davidson asked an exasperated Ms Sturgeon whether she could restore Ms Thomson to her job as business spokeswoman in the Commons "on the basis of ethics and integrity alone".

What the criminal law makes of these property deals we'll have to wait and see. But Ms Thomson is in clear breach of Mr Wishart's law.

The affair has also raised questions about the SNP's vetting procedures.

Prospective candidates are routinely asked whether anything in their background could embarrass the party. But should officials delve more deeply?

It appears the party was happy to pin its rosette on her on the basis she was a leading light in the Business for Scotland campaign group and on a vague understanding she was involved in property.

Senior ministers may now be regretting some of the glowing election endorsements they provided before the election, including Social Justice Secretary Alex Neil, who said Ms Thomson understood "how business can be used to support social justice".

Mr Wishart was speaking before the election which saw his party's complement of six MPs swell to 56. Many of the new MPs, not just Ms Thomson, must have missed his call for them not to have outside financial interests. No fewer than 15 of the remaining 55 have registered property interests with the Commons authorities. All within the rules - but it makes it harder for the SNP to portray itself as a party apart.