SLOWLY, we are getting to the point.

There is no longer an argument over whether Scotland should have greater autonomy. The only question remaining – a question full of intricacies and implications – is how much.

The ground is conceded, in any case. One by one they troop north to talk of more powers, never of fewer, for Holyrood. David Cameron, Alistair Darling, Ed Miliband, Nick Clegg, Douglas Alexander: each has an offer, or the suggestion of an offer, from the lucky bag labelled "fiscal autonomy". If you can keep the image of the late Alec Douglas-Home out of your head, it could pass for the spirit of co-operation.

A little caution wouldn't go amiss, however. Younger Scots might need to be reminded that in 1979 Lord Home of the Hirsel, as he became, solemnly promised lots more autonomy if we rejected what was then proposed. At the time, it was big news. But that grandee's notion, like the credibility of Scottish Tories, was soon forgotten.

Most of us should meanwhile remember that the Scotland Bill, the one that was supposed to fix all problems, is still crawling through its committee stage at Westminster. Has this supposedly groundbreaking piece of reforming legislation become redundant before it is even enacted? More to the point, who speaks for it now?

Hence the question we should all bear in mind. If Unionist leaders have become so free with their promises of what might be "on offer" – a formulation that misses the point entirely – why won't they put just one of their schemes to the vote?

You could rephrase it. If each of the Unionist parties accepts the SNP won a mandate for a referendum, how can they reject the holding of a vote on something that is actually less than independence? How can they reject a vote on the very ideas they seem, all of a sudden, to espouse?

It's not my job to offer tips to Mr Cameron, Mr Miliband or Mr Clegg. I wouldn't necessarily want to help them raise their game. It seems obvious, nevertheless, that these Unionists are painting themselves into a corner. Who solicits an informed electorate and says, "I have a better offer, but you can't vote on it"?

Instead, we hear Mr Cameron talking of clarity, of Mr Clegg asking how an independent Scotland would "work" – in contrast to his own coalition? – and of Mr Miliband resurrecting the old British Labour idea that social justice can only ever be attained within boundaries drawn by the Labour Party.

Yet on Radio Four's Today programme yesterday morning Johann Lamont, Mr Miliband's Scottish leader, struggled even to explain why a vote on one version of autonomy (independence) must be held forthwith, while the other (devo whatever) is – I'm paraphrasing – not for the likes of us. One vote good; two votes bad: it's not the quintessence of democracy. Worse, for Unionists, it makes no sense.

The polling evidence only illuminates their confusion. According to a survey commissioned by The Times – where Rupert Murdoch tweets for Scotland – 71% of respondents are now behind the Unionist leaders and their offers of increased autonomy. The idea, whatever it means, is hugely popular. The difficulty for those leaders is that 59% of the same sample have the strange idea that they are entitled to a vote.

On a narrow definition, and at this rate, Alex Salmond can't lose. His opponents want to believe that independence is, for him, win or bust. They want to depict the argument in those terms. But the First Minister will tell anyone who is prepared to listen (sometimes to their exasperation) that he is, in all things, "a gradualist". The word, like the process Mr Salmond favours, is conveniently vague. Its practical meaning is that he will take what he can get: more means more.

Much of the argument, on either side, does not make much sense. Take "devo plus", the latest flavour of the week. Unionist support, at least of the rhetorical kind, seems to be gathering behind this version of autonomy. Yet how, as Mr Clegg would say, might it work? Holyrood would have control of income tax, corporation tax and most elements of welfare spending. Holyrood would have no say, meanwhile, over pensions, VAT and National Insurance.

Try running an economy on that basis. It would ease the problems of the Treasury, HMRC and cross-border employers, no doubt, but it would not enhance economic autonomy – the right of self-governing choice – in the slightest. It would simplify life for Westminster and complicate matters, to no obvious benefit, for Holyrood. Clarity, the great Unionist watchword, would not be achieved.

But turn the issue on its head: would Mr Salmond accept such a deal? He would reject it publicly on the grounds of incoherence, no doubt, and maintain, of course, that nothing less than independence will do. But if there is nothing else available?

In practical terms it would be no worse than submitting to the apathy of the Bank of England, or taking your chances within the sterling area under self-determination, SNP-style. Mr Salmond's gradualism is an elastic concept. Much of the time he is not even talking about independence in any conventional sense. He too is haggling over degrees of autonomy.

If that's the case, the First Minister is also liable to wind up in a corner with a paintbrush in his hand. Pragmatism has become his default position. Rather than explaining how Scotland would be transformed by independence, he has made a habit of describing the things that would remain the same. He thereby risks alienating those disinclined to vote for an "independence" founded on the monarchy and the Bank of England. And he almost guarantees a vote for Unionist "autonomy".

Striking in all of this is a lack of a strategic grasp among those who would defend the Union. They have no vision, as best as I can tell, of an evolving United Kingdom beyond sops to dissent, bribes that might postpone a crisis, scraps to feed a hunger, and improvised schemes designed to avoid the issue.

So BBC Wales conducts a poll among a people who barely accepted minimal devolution at the second time of asking. The Welsh, it transpires, still do not incline towards independence. The figure in favour creeps up, from 7% to a mere 12%, only if Scotland is independent first. But do the people of Wales want more powers for the Cardiff assembly? These days, only 32% want things to remain as they are.

They will have their offer from London, no doubt, in due course. Unionist politicians sometimes seem to spend half their time traversing these islands fighting such brush-fires when they are not retreating into the Westminster bubble. They don't pause to ask: why are all these demands being voiced? Why are the structures of the United Kingdom failing in this manner? Why the need, the increasing need, to placate the provinces?

They could as well ask whether their efforts do more harm than good. Dispense a little of anything, autonomy not least, and people are liable to ask why they can't have it all. Some of the same people could also ask why Mr Salmond is strangely keen on a social union with what seems, on this evidence, to be a failed state.