HOW many nations was that, exactly?

Ed Miliband borrowed the tailcoat of Benjamin Disraeli this week to pronounce his faith in "one nation". Constitutional pedants (like me) might have pointed out that there are actually two nations in the United Kingdom, Scotland and England. Oh, and one of them might be about to leave.

Everyone in Manchester, from the leader down, now believes that a deal will be announced in the next couple of weeks on holding a single question, "in or out" referendum on Scottish independence in October 2014. "The dots have all been crossed [sic]", as one insider put it.

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We've been living with the technicalities of this plebiscite for so long that we've tended to forget the significance of it. In a couple years, the 300-year-old United Kingdom – one of he most successful unions in history – could cease to exist. This referendum represents a huge gamble by David Cameron: one that the Spanish premier, for example, is not prepared to take by giving Catalonia a referendum on secession.

But I'm still not entirely sure Labour fully appreciates the job it has taken on. David Cameron was responsible for agreeing the referendum on independence after the 2011 election, but it is Ed Miliband's Labour Party that has the responsibility of winning it, because the Tories are too unpopular in Scotland. Buttonholing Labour politicians in the Manchester conference centre, I did not get the impression that this is occupying their waking thoughts. Many seem remarkably complacent about the outcome of the referendum, believing that now Alex Salmond has been "put in his box" by the denial of his "second best" second question, the job is largely done. Few seem to realise yet that this single question has long been part of Yes Scotland's game plan.

Certainly, the opinion polls seem good for Labour Unionists. The big news is that there is no news. For 30 years now, survey after survey has told us that independence is the favoured option of only around one-third of Scots, and even many of them are unsure when questioned on the detail of the SNP policy. The vast majority don't want to leave the UK. So, on a single question – says Labour – how can they lose?

Easily. Labour in Scotland is in danger of making the same fatal mistake it made before the 2011 Scottish Parliamentary elections in assuming that a comfortable poll lead means it can take victory for granted. It can't. The shifting kaleidoscope of Scottish constitutional politics, and the SNP's reframing of independence as a kind of social union, means that it has to seize the constitutional agenda from the Nationalists, to prevent Alex Salmond persuading Scots to vote Yes to independence as the only way to ensure a better devolution.

The one politician who really seems to get this is Douglas Alexander, the Shadow Foreign Secretary. At a fringe meeting organised by the Institute for Public Policy research, he hammered home his message that Labour must put up a positive and radical "alternative story" to the SNP's and must offer "new possibilities" for Scotland's future. In other words, Labour has to take the wheel again and take the Scottish Parliament to the next level if it doesn't want to risk the SNP winning independence by stealth. As Professor John Curtice put it, for Scottish voters, "the default is home rule". The vast majority of Scots always say they want more powers for their parliament. Labour has no choice but to go with the flow, even it means letting Scots have their cake and eat it.

Certainly, there is no point giving lectures on how Scots can't "have it all", which is what the Scottish Labour leader, Johann Lamont, appears to be doing. She wants to take the cake away altogether. She has gone through almost the entire sum of policies achieved under devolution and dismissed them as "SNP bribes". Tuition fees, prescription charges, free personal care, concessionary bus fares – they're all part of the "something for nothing" society. But if you strip out these headline measures – most of them of course introduced by Labour – there's not a lot left to celebrate about the devolution decade.

This is daft politics, even if you believe that Scotland has been scrounging. Forget "Scottish solutions to Scottish problems": Ms Lamont's idea of "the common good" is to means-test prescription charges for the sick and make students pay £9000 university tuition fees. "This SNP government," she told Manchester, "is making the poor pay for the election bribes that benefit the better-off."

Someone needs to remind her that the people who suffer when these benefits are cut are the people who depend on them: working people earning on or around average household earnings. Rich people don't need bus passes, or free prescriptions or free tuition. If you means-test housing benefit or child benefit or prescriptions, it is always those at the margins who are hit hardest. Or have we learned nothing from 60 years of the welfare state?

Labour must refrain from this punitive language and turn positive. This is elementary politics. It needs to offer something along the lines of Devolution Plus (call it "Calman Plus"), giving Holyrood most taxes, bar VAT and National Insurance, and thus kill two birds. Labour will win the referendum, because this is what most Scots will vote for. And afterwards, Ms Lamont will be able to axe those "freebies", if such they are, because Holyrood will have to raise the tax revenues to pay for them. This implicit attack on devolution is playing into Alex Salmond's hands.

Yet, almost every day, developments like the rail franchise debacle, where the Scottish transport minister wasn't even told of the U-turn, demonstrate that Scotland and England are destined to co-operate. Decisions taken in Westminster are always going to affect Scotland – unless we decide to change trains at the Border. A form of federalism seems the obvious way forward for the partners on this small island. I think Scottish voters understand all this perfectly well. They are waiting patiently for their politicians to catch up. Only, like the trains, they sometimes take a long time.