IT'S a strange turn of events when the SNP Finance Secretary, John Swinney, takes to sounding more unionist than David Cameron.

Stranger still when Gordon Brown agrees with him. Last week, after the launch of the Tories' "English Manifesto", Swinney chastised the Prime Minister for failing to stick to the terms of the Smith Commission.

This was supposed to keep the UK together by continuing Scottish MPs' voting rights on UK taxation even though income tax is being handed to Holyrood. The logic of Smith is that since tax decisions in Westminster apply to the whole of the UK, Scottish MPs must still have a say on them.

But not any more. Under David Cameron's plans for English Votes for English Laws (Evel), Scots MPs could effectively be excluded from voting on the UK Budget which sets income tax. Indeed, English MPs would have an effective veto on most issues.

Scottish MPs have already become, in the eyes of the Tories and their press megaphone, aliens in their own parliament; accused of trying to "dictate" the government of England.

It was a week of bizarre paradox, reckless rhetoric, absurd contradictions. The Institute for Fiscal Studies became the Institute for Fiscal Confusion by criticising the probity of all the political parties in turn. And then was criticised in turn by all the parties for misrepresenting their policies.

But through the fog of statistical disputation, what became abundantly clear is that a significant section of Conservative opinion believes that Scots MPs should no longer have full voting rights in the House of Commons. And it appears that the LibDem leader Nick Clegg agrees with him.

There is no other way really of interpreting the near-hysterical claims that Nicola Sturgeon was intent on - and I quote - "holding England to ransom" by refusing to promise to vote for a Tory government. A more caustic attack on the sinews of union could hardly be imagined.

It seems that SNP MPs, duly elected to Westminster, are acting constitutionally only if they install David Cameron in office. If they vote against a Liberal-Tory Queen's speech - on the not unreasonable grounds that it is contrary to almost every clause of the SNP manifesto - the nationalists would be launching a "coup" on England.

The claims about a secret "pre-nup" deal between Ed Miliband and Nicola Sturgeon are nonsense. For their own reasons, Labour and the SNP are making a point of NOT speaking to each other before the election. They don't need a deal. All they need to do is allow the arithmetic to work.

As it happens, Labour look in a very good position to become the largest party after this election. But even if it is not, here is how the post-election equation could be resolved:

David Cameron as the sitting PM has the right to seek to form the first government. The Liberal Democrats, for reasons comprehensible only to Nick Clegg, have indicated they will try to repeat their toxic coalition with him and are suggesting they would not enter any coalition that was on "life support" from the SNP.

He's talking nonsense and here's why: On present polling the LibDems are in line to return around half the number of SNP MPs, around 25. But the Tories can also draw on maybe eight Ulster Unionists and a Ukip or two.

Cameron presents his legislative programme and invites parliament to endorse it. However, as soon as it goes to a vote it would almost certainly fail because Labour, SNP, Green, Plaid, SDLP and the Uncle Tom Cobley Party refuse to support it.

Government falls.

Because of the Fixed Term Parliaments Act 2011, there cannot be another General Election unless two-thirds of the entire House of Commons vote for one. Which they won't.

Labour will then put forward its legislative proposals and invite the Liberal Democrats to join with them in a "Grand Coalition of Stability" or some such phrase. It is inconceivable that the Liberal Democrats would refuse even to discuss a Lib-Lab coalition.

The LibDems demand no referendum on Europe, plus no £30 billion Tory cuts, a mansion tax, home rule, reform of the Lords, a legally enforced carbon emission target, 100,000 homes, fewer free schools (in England); protection of NHS from TTIP etc.

Labour squirm in anguish - and then accept these conditions in full, since they are also in the Labour manifesto. They add the 50p tax band which is popular among the LibDem membership. LibDem leader-in-waiting Tim Farron gets to be become Deputy Prime Minister with Vince Cable as Secretary to the Treasury.

Labour put this package to the vote in the Commons and - surprise - the SNP vote for it, on the grounds all of the above are also in their manifesto. Will the LibDems then say "we are going to withdraw now because this is on life support from the SNP"? Of course not. They would be defying the will of Parliament, the core of our democracy.

Conspicuous by its absence from this scenario is any negotiation whatever with the SNP. A stable UK coalition government has been formed in Westminster, with enough puff for a five-year term, and nary a mention of the Nats.

The Tories would cry foul and say this is a Sturgeon "coup" but that would be ludicrous. The SNP have done nothing but vote for their policies.

Now, I'm not saying the SNP will sit idly by on the backbenches throughout. They will make noises about Trident, austerity, fiscal autonomy and many other things.

They claim credit for making the deal work and locking out the Tories. And they will be correct to do so. They will be the saviours of this Labour government. Yet they need play no active part in it.

Some nationalists friends have been

rather put out by this scenario because it suggests the 50 SNP MPs (if such there are) would not have any power over the hated Red Tories. They shouldn't be. It is credit enough for the SNP to be responsible for ensuring the next government of the UK is not Conservative.

SNP MPs would be able to play a very important moral role as the social democratic conscience of Labour. They will be able to put forward motions rejecting the renewal of Trident, which some Labour MPs might support. Though in the end, the Tories and most Labour MPs will always unite to ensure that the nuclear deterrent is retained.

But that's good politics for the SNP. In 2016, they will be able to say: "Look, we tried. We kept the Tories out. But in the end, in Westminster Labour and the Tories will always gang together."

The Tory plans for Evel - English Votes for English Laws - would effectively write Scottish MPs out of the UK Budgetary process. It would effectively give English MPs a veto on a wide range of UK legislation and make it impossible for Scottish MPs to be Chancellor or Prime Minister.

Smith would be toast. The Tories are ripping apart the very unitary UK parliament they supposedly fought the referendum to save. The Barnett Formula could not survive.

So there's the final irony of this bonkers week. Evel will propel the constituent parts of the UK apart faster than full fiscal autonomy ever could. The SNP are pledged to lock out the Tories. The conclusion appears to be: vote SNP to save the Union.