ALL the pro-Union parties have offered more powers for the Scottish Parliament in the event of a No vote in September, but much of the discussion and analysis so far has focused on the differences between the plans, which are significant.

Now there appears to be the prospect of the parties coming together and making a joint declaration on increased powers that would amount to a guarantee that something, at least, would be delivered after September 18.

Exactly what form such a guarantee would take is not clear and any joint statement would certainly not tell us precisely what the new powers would be. This means a three-way guarantee of more devolution would probably not be hugely significant in terms of the process of constitutional change in the UK, but it could be significant in the ongoing debate ahead of the referendum and the ability of the SNP to claim that the only way to guarantee more powers is to vote Yes.

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In particular, a joint guarantee by Labour, the Conservatives and the Liberal Democrats is designed to shore up the idea that there would be more powers in a Scotland that stays in the UK but also puncture the persistent claims of the nationalists that the pro-Union parties are not to be trusted.

That was the message that the First Minister Alex Salmond was delivering again yesterday when he claimed that Scots could not rely on the unionist parties' promises. "The only guarantee of getting more powers is to vote yes," he said. "Anything else is in the grace and favour of the unionist parties."

There will be many voters who feel inclined to believe this and feel cynical about pro-Union promises, particularly promises from the Conservatives. The leader of the party in Scotland, Ruth Davidson, says Holyrood should be given full income tax powers but it was not so long ago that she talked of a line in the sand on devolution, a line that has now been scrubbed out. In the case of the Labour party, the cynicism about greater powers is much harder to sustain because Labour has an excellent track record of promoting and delivering devolution in the UK.

All the pro-Union parties, however, recognise that the danger of voter scepticism about their promises is real and a joint guarantee of more powers is their way to tackle it. They also recognise the fear that more devolution after a no vote could come to grief in a lack of consensus about how far that devolution should go. This danger is real given the differences between the parties, particularly on tax. Labour has held back from offering full control of income tax largely because they worry a Nationalist government would use it to cause trouble for Westminster.

While there is no prospect of the pro-union parties reconciling these differences in detail before September 18, they do now agree on the principle that a post-no Holyrood must have more powers and be more accountable for them.

In some ways, a joint statement does not take us much further forward - political promises can be broken - but a joint guarantee of more devolution would make it much harder for the pro-Union parties to backtrack without sustaining significant political damage.