Referendums are like buses: you wait three decades for one and then four come along at once.

That Scotland voted to stay in the European Union but England and Wales voted to leave was always going to punch that particular constitutional bruise. If Brexit does go awry, the prospect of leaving one union to rejoin – remain within – one union becomes more attractive.

Now Theresa May is braced for a second referendum on Scotland’s future, not after but during Britain’s exit talks, the Times reports.“Scotland to demand new referendum, No 10 fears” is that paper’s Ronseal splash. The story is already making itself felt on the currency markets, with sterling down yet further against the dollar at time of writing.

The PM’s options aren’t good. Although notionally the right to hold a referendum is power reserved to Westminster, the prospect of the elected government at Holyrood asking for another only to be refused by the Tory in London is the worst thing that could happen to the Union since, well, Brexit. In any case, there is nothing to stop the SNP holding a non-binding, Scotland-wide consultative poll, putting further pressure on the constitutional settlement between Scotland and the rest of the United Kingdom.

Let’s say she decides that the best thing to do is go ahead with the contest. A lot has changed since 2014 and the No side's victory. Back then, Ruth Davidson said that a Conservative victory in 2015 was not “likely” and one of Better Together’s key themes was that a Yes vote would put Scotland’s EU membership at risk.

Now all the signs point towards a Tory victory that effectively rules out the chances of a Labour revival in 2025 as well, and Scotland’s EU membership is gone.

Then there’s the Irish dimension. Northern Ireland and Scotland’s constitutional affairs are not the same but in any referendum held during the European talks, the unionist side will have to explain why they are talking up the prospect of a open border between the North and the Republic while warning against a hard border between England and Scotland. What’s good for peace in Northern Ireland and for maintaining that bit of the United Kingdom is not good for the other.

That’s before you get to the questions of who would lead it: Labour are unlikely to want to get back on that particular train and in any case are not the force they were in 2014, to put it mildly, while a No campaign headed by a Tory feels like Nicola Sturgeon’s dream.

But equally it’s not as easy as it looks for the SNP. A second No campaign would go hard on difficult questions about EU budget rules, the Euro, and exports to the rest of the UK. In addition, questions about pensions (at risk) and immigration (likely higher than now) would all be weaponised in ways they weren’t before. Indy Ref 2: Indier Reffier would be an uglier affair than the contest that came before. Those Yessers to the SNP’s left are less inclined to fall in line with the big beast of the Yes side, too.

All in all, it would be a harder and more gruelling contest for both sides. Which isn’t to say that the SNP’s chances wouldn’t still be significantly higher.