THESE are heady days for John Buttifant Sewel. Three years ago, the former Labour minister quit the Lords after featuring in a jaw-dropping tabloid expose about his private life. That bra really never suited him. But now he’s the toast of the political steamie. The BBC even tracked him down for on interview on the convention that bears his name.

Yes, he thought Brexit justified the UK Government departing from it. No, he didn’t think there was a constitutional crisis. His comments were awkward to the SNP’s plan to bury devolution, but not fatal.

Despite the convention’s architect declaring it structurally sound, SNP ministers carried on warning people to take cover, shrieking it was about to topple at any moment.

It’s 20 years since that famous bit of non-law came into being. On 21 July 1998, Lord Sewel and his fellow peers were debating the Scotland Act, which would give us devolution and the Scottish Parliament.

In particular, they were debating the clause asserting there would still be no limit to “the power of the Parliament of the United Kingdom to make laws for Scotland”.

As devolution was about Scotland making its own laws, Westminster’s role would, however, be constrained by a self-denying ordinance. As Lord Sewel said: “We would expect a convention to be established that Westminster would not normally legislate with regard to devolved matters in Scotland without the consent of the Scottish Parliament.”

If problems arose, they should be fixed “through political dialogue” and if there was an impasse “there is the ultimate fallback of… changing the devolved powers”.

He did not say Westminster would never legislate in devolved matters, nor always concede in the event of a deadlock. Far from it, he said if there was an impasse Westminster could always “revisit the whole issue of the devolved settlement” and go back to legislating for Scotland.

He went on: “This Bill is about the establishment of a stable and devolved settlement within the Union. On that basis sovereignty rests with this Parliament. There is no escape from that and no attempt to duck it. There is no possibility of having any form of dual sovereignty. That is not a concept that is capable of any rigorous scrutiny.”

Lord Hope of Craighead, one of Scotland’s most senior judges and later deputy President of the Supreme Court, added: “The Parliament at Westminster cannot abandon its own sovereignty. It is absolutely fundamental to the whole arrangement that the Parliament at Westminster can take away what it gives at any time.”

Such was the thinking behind the Sewel convention. It was not meant to give Edinburgh the upper hand, or even an equal one, but rather to minimise Westminster’s role.

Yet the SNP are now rushing to defend this Unionist device in light of Brexit. The UK Government’s unprecedented decision to press ahead with the EU Withdrawal Bill despite Holyrood withholding its consent has knackered Sewel good and proper, we’re told. Until it is fixed - and by this the SNP mean made “legally binding” - the Scottish Government will be withholding consent for all the other Brexit-related Bills coming its way for approval under the convention.

Brexit minister Michael Russell wants to embed “the requirement for the Scottish Parliament’s consent in law”, on devolved matters, or it’s hardball and fights from now on.

As I wrote last week, the SNP strategy between now and the 2021 Holyrood election is to argue that devolution is on its deathbed under the Tories and needs replaced with independence. Mr Russell didn’t go quite that far yesterday, but he did call devolution “broken” and its operations and structures “failed”.

Is it broken? Holyrood has passed more than 170 legislative consent motions under the Sewel convention since 1999. It has withheld consent only twice - a dispute over welfare in 2011 was resolved - and only on the Withdrawal Bill has Westminster legislated regardless, citing the ‘not normal’ circumstances of Brexit. A single instance is not a pattern.

As Lady Hale, president of the UK Supreme Court, said last week about Sewel: “A constitutional convention is… supposed to be the universal practice of politicians, otherwise it isn’t a convention. But that’s a big debate. Sometimes conventions are departed from. In fact, quite a few of them are departed from.”

That said, a convention’s value is proportional to its observance. If more breaches occur, Sewel’s shelf life will be in doubt. But for now, I would say SNP claims it has been ripped up are wishful thinking.

The really interesting bit is Mr Russell’s proposed solution. It is impossible, and he knows it. We can be sure because the same issue is at the root of the ‘power grab’ row over the Withdrawal Bill.

In that case, Holyrood wanted its consent to be embedded in law for UK-wide common frameworks in devolved areas after Brexit. The UK government refused, saying that would give Holyrood a veto power over legislation outside Scotland. The refusal was also informed by constitutional arguments over Westminster sovereignty.

Now Mr Russell is making the same demand on a vastly increased scale. He wants a veto power over all UK legislation covered by Sewel convention - that is, legislation that touches on devolved areas, Holyrood’s legislative powers, or the executive powers of Scottish ministers. That’s a lot of law. Most would be uncontroversial, and Holyrood’s consent doubtless granted.

But the UK government is never going to give another legislature - let alone one dominated by its political enemies - a stranglehold over its legislative programme. Nor could the current government bind its successors to accept such a deal. And, as the Lords debate in 1998 and more recent ones over Brexit have confirmed, it is likely to be impossible on a constitutional level too. Westminster’s sovereignty cannot bear any encroachment.

The SNP is thus presenting Tory ministers with an unwinnable dilemma: convert Sewel into law, or keep over-ruling Holyrood on Brexit-related Bills and become the wreckers who threw devolution under the bus, and potentially the midwives to independence.

It has taken two years of pushing, and not a few missteps, but the advantage is now with the SNP.