Today’s correspondence between Boris Johnson and Nicola Sturgeon was as terse as it was predictable.

The Prime Minister confirmed that the British government would “continue to uphold the democratic decision of the Scottish people and the promise that you [Sturgeon] made” to accept the verdict of the 2014 referendum as binding for a generation. Sturgeon replied (on Twitter) that this was proof that the Tories were “terrified” and therefore attempting “to deny democracy.” No love lost here. And no surprise, either.

This official spurning leaves Sturgeon with two options. The high stakes option is to “do a Catalonia” and call a referendum in Scotland without legal sanction from Westminster. The alternative is to wait until the May 2021 elections to the Scottish Parliament give the Scottish National Party the majority it can, and will, interpret as a mandate for a second referendum.

There is no legal avenue by which Holyrood can circumvent Westminster’s veto on this issue. That is the clear finding of yesterday’s paper by two leading legal academics, Chris McCorkindale and Aileen McHarg.

Ignoring the law presents obvious problems. One look at what has happened in Catalonia is enough to scare off all but the most impatient Nationalists. The British state might not respond with the vigour of its Spanish counter-part in jailing the leaders of an unconstitutional vote. But, Sturgeon recognises the danger inherent in attempting to hold an unofficial poll (even dressing it up as an ‘advisory exercise’ – which Unionists would torpedo by boycotting it as an expensive PR exercise unworthy of their participation).

What is the alternative? By the spring of 2021, not only will Brexit be last year’s news, even some manner of UK/EU trade deal may have been concluded. If either or both can be blamed on a noticeable downturn in the Scottish economy, then the turn of events will be propitious for the SNP’s push for IndyRef2.

But what if most Scots experience no clear detrimental effects from Brexit? Even a Remainer as passionate as Scotland’s First Minister must concede this possibility in her calculations. No wonder Sturgeon wanted the vote now when uncertainty is at a premium – and before Alex Salmond’s trial on charges of serious sexual offences commences this coming March.

Given that Johnson won’t play ball, how much time should Sturgeon bide? Even if in May 2021 the SNP wins a stonking majority at Holyrood with a manifesto explicitly demanding an IndyRef, the Johnson Government has made clear that it will not accept this as a mandate for agreeing a new referendum. What then?

One option will be to manufacture some form of crisis in Scotland that creates such a threat to civil order that a frightened Westminster decides to let the air out of the radiators with a second IndyRef. For so long as the SNP play by the rules it has only one other route – to wait until there is a Labour or minority government returned to Westminster. Which will not be during this decade.