It’s an odd manifesto pledge: “We guarantee to lose the country 11,000 jobs.” But there is an even odder one to come. Alex Salmond, the SNP leader, mumbles about having diesel-powered submarines instead. I haven’t checked on eBay recently, but I suspect they are in short supply. The USA has stopped making them, and any new ones would have to be specially ordered from Germany, which still nurses the technology; so an independent Scotland would be losing Scottish jobs and creating German ones.

There is also the whiff of hypocrisy here. Mr Salmond knows full well that Scotland would continue to shelter under the umbrella of a Trident-defended England, rather as Canada benefits from the USA’s nuclear force.

Defence is just one of the absurdities of this sudden fad for Scottish independence. The arguments come from left and right, from north and south of the Border. They assume that devolution is untenable and the break-up of Britain inevitable. To listen to them, you would assume that the border posts are already being constructed, the constitution redrafted and the genesis of an English Parliament to head off the Scots is only a Commons vote away.

Where does it all come from? The SNP is running ahead of Labour in Scotland by about five points, and there is said to be a narrow majority (52-48) in favour of independence. We have had rumblings of English discontent over the rights of Scottish MPs at Westminster and the prospects of a Scottish prime minister. There has been resentment over the generous subsidies that Scotland enjoys. Some maverick Conservatives have been arguing for independence as a way for Scotland to rediscover its entrepreneurial past.

All this, it is said, suggests that a Union that celebrates its 300th anniversary next year has run its natural course. “Now, there’s ane end of ane old song,” said one of the commissioners who negotiated the treaty in 1707. Did he really speak too soon? I doubt it. The idea that a revolutionary wind is blowing through the nation of Bruce and Wallace, fluttering the standards and readying the people for freedom is illusory. There is discontent and even disillusion with the Parliament, which has yet to live up to the heady expectations of 1997; but the SNP has not even begun to demonstrate its credentials and, above all, its ability to run an economy that enjoys a £30 billion slice of the national cake.

As David Aaronovitch pointed out on these pages last week: “The argument for Scottish independence is essentially an argument for avoiding hard choices.” The hard choice will come when the SNP’s generous commitments on hospitals, schools and student grants are costed, and the tenuous argument that oil revenues will close the gap without the need for serious tax increases is put to the test. Nothing in the polling data suggests that there is any sudden conversion to the idea of a separate nation. With five months to go before the Scottish elections, they reflect something similar to the Tory lead over Labour in England. The SNP in Scotland is the official opposition, and though it often does well in polls before elections, it tends to fall back again when the time comes to vote. It is, in short, an end-of-term verdict on a less than inspirational government.

The really hard choice is about how to build confidence in a parliament and a system that is now seven years old and is still absorbing the seismic constitutional shift that led to its creation. The remarkable thing is not how little it has achieved, but how much has been done without straining the relationship with Westminster.

Despite separate policies on tuition fees, free care for the elderly, the NHS and schools, which have taken the country on a course as divergent from Mr Blair’s programme in England as it is possible to imagine, there has been none of the friction or distemper that was predicted. Bills now progress through the Scottish Parliament monitored not by a second chamber but by a committee system that itself is new, and that largely works. We have two parties in coalition — a system that works — and now there is to be proportional representation in local government as well. All this has been absorbed, grumblingly but smoothly, and almost unnoticed in England.

The idea that this should be thrown overboard in favour of separation seems absurd — and I predict that the Scots will feel the same when it comes to election time. They may vote for the SNP, but they will do so not as an independence party but as an opposition. They know that it is committed, if elected, to holding a referendum on independence within its first term of office. That referendum would almost certainly be lost. Watching Trident jobs and the insurance policy of an independent deterrent drifting south to England is not exactly a vote-winning proposition.

The Scots would look hard at the prospects, the increased taxation, the uncertainty of oil revenues, the quality of the SNP’s own ministers, the lunacy of its defence policy and, above all, the separation from a partnership with England that they have enjoyed and benefited from for more than 300 years, and will say: thanks, but no thanks.

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