PROFESSOR Andrew Goudie, the former Scottish Office chief economist and one of the most widely respected observers of the independence debate, was scathing by his own restrained standards about the pro-UK parties' approach to devolving further powers to Holyrood in the event of a No vote.

In a paper for the Fraser of Allander Institute this week, he argued their failure to offer a common, agreed package of powers raised a "serious question of credibility".

Can they be trusted to deliver he asked. Given the significant differences between the parties on a range of issues, not least their plans to hand Holyrood greater control over income tax, what might an eventual compromise look like?

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"It is surprising," he concluded, "that the pro-Union group has managed this issue so poorly and in such an incoherent manner. Convincing a highly sceptical and suspicious electorate will be a massive task."

The question of what will or won't or might happen if, as the polls continue to suggest, Scots vote No in September, is becoming a bigger part of the debate by the day. As Professor Goudie says in his paper, it could have a huge impact on the referendum.

The Yes camp's argument that because promises of devolution were made and broken in 1979 the same will happen in 2014 seems less than convincing, especially when it is advanced by politicians sitting in a parliament that was promised in 1997 and opened for business in 1999. However, the idea that a No vote, especially a comprehensive one, might put the brakes on further devolution cannot be brushed aside. Might the parties' apparent urgency wane? Might their conflicting proposals be boiled down to a lowest common denominator in the name of achieving consensus?

In weighing this up, voters will have to consider a couple of things. Firstly there is the political damage Labour, the Conservatives and LibDems would suffer if they failed to meet the public's expectations. Johann Lamont, Ruth Davidson and Willie Rennie's recent day out on Calton Hill in Edinburgh was thin on substance but at least served to paint a target on their backs if, in the end, they were to back-track on the more-powers agenda.

The second point is usually overlooked. It's this: if they win, the three pro-UK parties will not want to go through another independence referendum process for a very, very long time. Preferably, never again. That's why there is a growing feeling that a deeper, longer-lasting constitutional settlement that would satisfy a clear majority of Scots (and people in the rest of the UK) must be found.

The notion was articulated this week by, of all people, Sir John Major. A staunch opponent of devolution in the early 1990s, the former prime minister said he was "relaxed" about further powers for Holyrood provided the impact on the rest of the UK was taken into account. A way to answer the West Lothian Question must be found. He is not the only Conservative thinking that way. The Strathclyde Commission, which drew up the Scottish Tories' devolution plans, acknowledged much the same, though considering possible options fell outwith its fairly narrow remit.

Labour, too, are looking at a bigger picture. The Herald reveals today that Gordon Brown's proposal to replace the House of Lords with a US-style Senate representing the nations and regions of the UK equally, is being considered as part of Labour's election manifesto process. If it is agreed (still quite a big "if" at this stage) it will be part of much wider programme of constitutional and democratic reform than simply handing Holyrood specific tax powers. Also in the mix are plans to empower the UK's "city regions," outlined by Ed Miliband earlier this year as a way to satisfy regional demands for greater control over local economies and to offset the increasing dominance of London. "Often we see devolution through the wrong end of the telescope," a Labour source said, referring to the inevitable, limited focus on Holyrood's powers. The pro-UK parties are considering a much bigger democratic shake-up than simply letting MSPs set your income tax.