From what you hear, Labour has performed another handbrake turn on what it sometimes calls a devolution journey.

What is less clear is who is at the wheel. As to where they think they're going, no one can rightly say.

What we do know is that the party long in the slow lane in the Holyrood more-powers demolition derby is motoring - or claims to be motoring - to catch up. There's no mystery about that. Crushing opinion polls, mind-boggling SNP membership figures, a shambles within the party: this is a race against the clock.

So a "U-turn" is reported. The party that has turned humming and hawing over income tax powers into mouth music since its devolution commission reported in April last year is resolved. If tomorrow Lord Smith of Kelvin advocates full Holyrood control over rates and bands, Labour will not demur. Apparently.

There's a tiny mystery in this. If a decision has been made, by whom has it been made? Ed Miliband has not spoken. When last we heard, Ed Balls, shadow Chancellor, was supposed to be dead against this sort of tax devolution. Gordon Brown, heading for the exits, has pronounced it anathema. A majority of Scottish Labour MPs have been the opposite of keen.

It is possible, of course, that panic and opinion polls have caused all of the above to change their minds. But it is only a couple of days since Alistair Darling said in the Financial Times that "floods of tears" could follow the very reform Labour is now - reportedly - prepared to endorse. So who has authorised the volte face?

Contrary to the impression being given, Scottish Labour does not have a leader at the moment. Ordinary members might have been persuaded otherwise, but the party does not even have a leader-in-waiting. Iain Gray and Gregg McClymont have been negotiating away at the Smith Commission, but on whose behalf?

Mr McClymont, the Cumbernauld MP, is shadow pensions minister and would therefore expect to hear Mr Miliband on the phone now and then. The rest of the chain of command is obscure. Yet there we have it, reported widely: still another change of tack on the income tax question. As if by osmotic magic, "Labour" has decided. In the absence of a Scottish leader, do we take it that means London, as usual?

Funnily enough, every report on the great U-turn has involved a confusion between the party and a candidate for the Scottish leadership. Jim Murphy, the East Renfrewshire MP, gave a speech at the Mitchell Library in Glasgow yesterday in which he claimed the new income tax policy as his own. He informed his audience, indeed, that he had discussed the matter with Mr Miliband and Mr Balls. Do Neil Findlay and Sarah Boyack, the other candidates, enjoy such privileges?

Scottish Labour members might want to think about this. An MP who has yet to win election to the leader's job appears to be dictating party policy towards the findings of the Smith Commission. The U-turn - or Murphy Manoeuvre - is all but set in stone. A figure who does not sit at Holyrood is charting a course for Labour in Edinburgh. A decision from the party's electoral college seems like an afterthought.

Big surprise, you might say. After all, it wasn't just the Unite union that took a keen interest in organising a candidate for Falkirk. It's the Labour way. Mr Murphy has given us his opinion on tax devolution and 50 per cent rates and managed to sound as though the party's official reaction to Smith will be a mere echo of the choice he has already made.

That might well be the case. But what kind of choice is it once you set aside the leadership ambitions, the vow-keeping, and Labour's desperate need to avert disaster next May? Each of the parties involved in the Better Together campaign to preserve the Union could be asked the same questions as they hurl concessions into the devolution maw. Will this work? Where, exactly, is it supposed to lead?

Where income tax is concerned, Mr Darling and Mr Brown have been perfectly clear. Full (almost full) devolution of the tax would put a giant hole in their "pooling and sharing" rhetoric. Since income tax amounts to 27 per cent of UK government revenue, the loss of control would make a Chancellor's job next to impossible. As Mr Darling said in the FT, "for the first time in 300 years, a UK government would no longer have control over its power to raise income taxes".

That's not a problem for Scotland, necessarily, unless (or until) changes to the Barnett formula are imposed, or proper powers to borrow as required are refused. No matter what, it would certainly lead to every Budget becoming an "English votes for English laws" argument. How could any Scottish MP possibly vote on taxation in England? How could any Chancellor begin to balance his books?

Parallel problems arise with the benefits system. Willie Rennie of the Liberal Democrats and others have been talking up a "significant package". Details remain mysterious, but it hardly matters. Iain Duncan Smith's universal credit would be a still-bigger shambles. The integration of tax and benefits within an economic union would become a laughable notion.

The closer you get to fiscal autonomy for Scotland, the less chance there is of either part of the Union being "better together". That's fine for the SNP. They get powers that are adequate, or they point out inadequacies. They get powers that connect in a coherent way - the key issue - or they say again and again that the concessions are a mess. If Scotland is penalised at any point, the SNP's response writes itself.

Those such as Mr Murphy - and all the Unionist parties - are attempting to satisfy contradictory demands. They want to keep the UK together as a functioning, united whole. The opinion polls say they must also placate all the voters demanding "more powers". Where money is concerned, where tax and benefits systems are at stake, it can't be done in any way that will ever make sense.

Besides, Nationalist politicians have another old card to play. It goes like this: if power X, why not - once the principle is conceded - power Y? If Holyrood is to have charge of the income tax raised in Scotland, why not, the EU permitting, any revenue raised in Scotland? All the signs on this road say that autonomy is a one-way street. When "pooling and sharing" is revealed as a dismal fiction, the game's up.

Those who won September's referendum no longer care to hear about that. Avoiding a rout in May, and again in 2016, is their sole obsession now. Mr Murphy, for one, is marching towards what Mr Brown has denounced as "a Tory trap". Labour voters choosing a new leader can decide for themselves what they think about that.

Devolution has limits, or it leads to independence. So which Unionist leader is brave enough to draw another of those lines in the sand? Not a one, this winter. Not before the public's gaze, at any rate.