ONE thing in Scottish life is certain.

Should the SNP ever come up with a plan B, some opponent will spring forward to condemn the reckless failure to prepare plan C. It's in the nature of the game. And it is not a game the party of government is supposed to win.

The people who make the demand for a policy alphabet itinerary want the sort of news from the future that even Doctor Who would struggle to supply. They can't or won't tell you where Scotland will stand in the event of a No vote next September, or even, with any clarity, what they have in mind. They can't tell you what 2015's Westminster elections will bring. If they once did a chancellor's job, they will hesitate over long-range economic forecasts.

But they see it as their task, indeed their duty, to demand cast-iron guarantees of the SNP. One reason is the No campaign has been constructed around the idea of uncertainty, a word presumed equivalent to rank fear among voters. Another reason, sound enough in a short-term political campaign, is they want to pin Alex Salmond and his party to a single position, the better to pick it apart.

We have witnessed as much this week in a series of choreographed attacks on the governing party over the currency arrangements it proposes for an independent country. I happen to think the proposal is a poor one, but we'll get to that. The No campaign doesn't much care. The SNP could choose any one of the three available options and Unionists would want to know about the beta plan.

As to the three possibilities, they are easier to state than to endorse. We could embark, as the SNP suggests, on a currency union with the rump UK, claiming our stake in the Bank of England and its decision-making. Mr Salmond believes London would grab such a deal, in the end, partly because of the balance of payments contribution to be had from Scotland's oil reserves. He also seems to think the pound's survival would calm voters' nerves.

Alternatively, we could embrace the euro. Interestingly, Unionists have stopped claiming an independent Scotland would be forced into such a move by "Brussels", perhaps because the strain of keeping two false ideas in their heads at once has been too great. The political reality is nevertheless straightforward: in the short-term, it's not going to happen. The eurozone might have defied all the prophets of doom, but it is still in a very bad way.

That leaves an independent currency for an independent country. Mr Salmond seems a bit muddled about this. If oil reserves make a sterling union attractive to London, why could those same assets not be used to back a Scottish pound? Exchange rates and transaction costs would present their usual problems, but several smallish European countries count them as prices worth paying. The political calculation seems to be, nevertheless, that this would involve more excitement than we can handle.

As mentioned, the No camp doesn't much care. It has nothing to say about what its members would offer if the referendum goes against them. A sterling zone is the target presented; that's the target for a rhetorical carpet-bombing. The operation depends on a fascinating notion of relationships within these islands. It proceeds from an assertion: don't expect any help from London. In fact, you will need that plan B because London will almost certainly screw you around.

George Osborne, the chancellor, has already asserted an independent Scotland would have to toe the Treasury's line. He has also doubted a rump UK would be interested in an arrangement involving inevitable compromises. Now Colin McKay, head of the Scottish Government's strategy unit has stated the obvious at a seminar in Glasgow: "We cannot assert as a priori fact that we can achieve a currency union with the UK, but we can set out why we think it is the best option."

For Alistair Darling, chairman of Better Together, that will do nicely. Scottish Labour's Johann Lamont and Alistair Carmichael, Scottish Secretary, cannot help but agree. The failure to provide guarantees for negotiations that have yet to take place are held to be lethal to the SNP's proposals. Surely the world remembers Mr Darling's plan B when the entire banking system was within hours, by his description, of shutting down?

Undaunted, the former chancellor said this week: "Mr Salmond's claim a currency union is a done deal has been blown out of the water by his own head of strategy." Almost simultaneously, by sheer coincidence, Unionists on the Commons Scottish affairs committee were also calling for a plan B and knocking lumps out of the Scottish Government's White Paper on independence, a document the committee has yet to read. But that detail was no obstacle.

Mr Darling has previously described a sterling area as a logical step. Was he speaking for his party and the attitude it would adopt if Labour wins the next Westminster elections? Is he saying Mr Osborne alone would be wrong to rebuff an independent Scotland? The answers have become opaque. Better Together's only interest is in attacking the lack of a fall-back plan and in forcing the SNP to describe the unsought thing.

This leaves aside the question of threats and menaces from London. Would some future chancellor really believe himself better off without a sterling zone? That would either be because he didn't trust a Scottish Government to manage its economy, irrespective of the usual pacts, or because he had no faith in Scotland's economic viability. The trouble is, smart Unionists have dropped the "too poor" argument. We could certainly be independent, they concede, but we would not be better for it.

The irony here is that, as it happens, an independent Scotland should not touch the Bank of England with a barge-pole. That institution has not had our best interests at heart in the past. It is unlikely to discover a common purpose with a foreign government, no matter how many committee seats Edinburgh might secure. The first duty of the central bank for the rump UK would be to the rump state. That's no basis for trust.

Ian Davidson MP, chairman of the Commons committee, has done his best to make this plain. The Yes campaign, he says, "cannot ignore or gloss over the risks and uncertainties that exist, the biggest of which is what the UK government and others can reasonably be expected to agree to in the negotiations". It is a strange sort of Unionism that gloats over London's refusal to contemplate friendly co-operation, but that's no problem when you are stirring up uncertainty.

Plan B doesn't have to be explained. An independent currency for an independent country is the only defensible solution. As the assaults on the SNP compromise subside, news arrives of a £4 billion investment in the Kraken field, promising 140 million barrels of oil and support for 20,000 jobs. It doesn't guarantee a future forever. But on such things a currency and a country can be built. London knows it, too.