WESTMINSTER'S strident refusal to share the pound with an independent Scotland continues to haunt the Unionist parties.

Since an unnamed Coalition minister last week torpedoed plans to reject a currency union by insisting that, "of course" there would be a deal after a Yes vote, the No campaign has been scrabbling to regain ground. However, the process behind Westminster's refusal is looking murkier still.

It has been alleged that Alistair Darling was the architect of the Treasury's hard line, as the chair of Better Together wanted to put a question mark over the currency. But the Treasury cannot say when its top civil servant first advised against a currency union. A paper trail a mile long might have been expected, given the political fallout that would come from this advice. But the Treasury has "no record" of the date it first arrived at such an unequivocal stance.

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The SNP smell a rat. The Treasury's formal advice, so helpful to the No campaign that it was made public after two days instead of the usual 30 years, appears to have been cooked up for political, not economic reasons, the party says.

It has a point and, in the interests of transparency, the Chancellor and Darling must explain the policy's lineage.

Fortuitously, Chancellor George Osborne and his Labour shadow Ed Balls will soon appear before the Scottish Affairs Committee at Westminster to discuss this very subject. It is a prime opportunity for the SNP to probe the issue deeper and secure much-needed answers.

However, the party has no plans to be there. Its representative, Dr Eilidh Whiteford, has boycotted the committee since October 2011. The reason is a spat with its Labour chairman, Glasgow MP Ian Davidson, who once warned her she would get a "doing" if she leaked any private deliberations. While not seeming a genuine threat, it was not an impressive interjection.

The SNP said it was menacing and carried an undertone of sexual violence. Davidson issued a minimal apology but Dr Whiteford has refused to return until he resigns.

Also, sceptics noted that the SNP's withdrawal came after the committee agreed to hold an inquiry into Scotland's referendum on "separation" - a process the SNP did not welcome.

And that is how matters stand. For two-and-a-half years the SNP has chosen to leave an empty chair rather than field an inquisitor. As the referendum nears, this looks like a self-defeating tactic.

If the SNP want answers on the decision-making process regarding vital issues such as currency union, the party should seize on every opportunity to interrogate those at the heart of that process.