There are significant differences between the pro-Union parties on plans for more devolution in the event of a No vote in September, but they do now agree in principle that a post-No Holyrood must have more powers.

Yesterday, the leaders of Labour, the Conservatives and the Liberal Democrats in Scotland jointly promised to increase devolution, particularly in the areas of fiscal responsibility and social security. "We believe that Scotland should have a stronger Scottish parliament while retaining full representation for Scotland at Westminster," they said.

But what does all of this mean in reality? Fine words have been spoken before on the subject, most infamously by the former Conservative Prime Minister Sir Alec Douglas-Home during the 1979 referendum when he promised a future Tory government would deliver more devolution - and we all know what happened to that.

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However, a joint pledge from all the Scottish opposition parties should be in a different league. There will still be some cynicism about pro-Union promises, particularly in the case of Ruth Davidson, the leader of the Scottish Tories, who once talked of a line in the sand on devolution, a line she has now willingly stepped over.

But the joint statement means that, whichever way the vote goes in September, Scotland should end up with significantly more powers. Holyrood already has the power to vary income tax by 3p (albeit a power it has never used) and, from 2015, it will have the power to vary income tax by 10p in the pound. Now we know that, whichever party, or combination of parties, wins power after a No vote, Holyrood will be even more financially accountable. When a large swathe of public opinion favours a much stronger parliament within the UK, that is a welcome move. It also establishes the principle that Holyrood should have more powers but should be more accountable for the power it wields too.

There remains a fear, however, that the promise could unravel if the parties fail to reach a consensus after a No vote and, indeed, there are big differences between them that could be hard to reconcile. For example, could the tax-lowering Tories support the Labour plans to give MSPs the power to increase the higher rates of income tax but not lower them? And could Labour support the Tory plans for Scotland to be given full income tax powers when they fear an SNP administration would use the powers to precipitate a crisis in the Union?

There will also be disappointment among some that the parties did not go further and unveil a detailed plan for more powers. Indeed, on the page opposite, Ben Thomson, chairman of Reform Scotland, argues that, if the parties had combined the best bits of their plans, they would have been able to point to a clear alternative for those considering supporting independence.

That detailed plan has not emerged. Even so, the guarantee unveiled yesterday is a significant step forward in a process of increased devolution begun in the 1990s. In principle, it is a guarantee that could appeal to those Scots who want more than the status quo but who have doubts about independence.