One referendum ends, another begins? The Article 50 Bill has passed through Parliament and Theresa May will trigger the process at the end of the month.

But Nicola Sturgeon has thrown a spanner into the works by announcing that she will seek a second referendum on whether or not Scotland should remain part of the United Kingdom.

A lot of people are guffawing about how the 2014 vote was "once in a generation" and that the SNP would have sought a referendum re-run regardless of the circumstances. And this is true, up to a point. They're the Scottish National Party, the clue is in the title.

But it's equally true that the SNP won the Scottish parliamentary elections on a platform of a referendum in the case of "material change" and while the consequences of Brexit are divisive at Westminster, I am yet to find anyone who doesn't believe it will represent a material change to how Britain has been governed. Yes, the SNP didn't win a majority thanks to their electoral system, but they triumphed with a bigger lead and a higher vote share than David Cameron got with his referendum promise in 2015. Thanks to the support of the pro-independence Greens, they will have the votes to get it through Holyrood.

The question is what happens then. The right to hold a referendum is reserved to Westminster, and the PM may well decline to extend that right. (There's the added subplot that a Referendum Act has to pass through Parliament. Don't forget that Cameron had a majority of 77 thanks to the coalition, while May has one of just 16. If she says yes to a vote, the path between that and a referendum act is thorny.)

As I explain at greater length here, May is in a lose-lose situation. If she says yes, she risks going down in history as the Prime Minister who presided over the break-up of the United Kingdom, and it is a real risk.

But if she says "no", she hands the SNP a drum to beat whenever the Brexit negotiations look to be going badly: we could be out of this mess if it wasn't for the Tory in London keeping us back.

But what should really worry the Unionist side is that they are having a procedural argument about whether or not there should be a second referendum and wasn't this all settled last time? The insistence that the SNP focus on the "real" issues of public services and the economy has the same familiar echo. It's all uncomfortably close to the position of pro-Europeans in the years before 23 June 2016. There are holes in the case for independence but they have a message and a sense of who their best team is. The same really can't be said for No.