The ironies are almost too painful to ponder. A British Prime Minister set on exiting a powerful and beneficial political and economic union – Brexit – lecturing a Scottish First Minister on the folly of separation. And a Scottish First Minister seeking to abandon its 300-year union with England but to attempt to remain a member of a different political “club”, the European Union. Theresa May is a leader who campaigned, albeit not vigorously, against the British self-amputation from Europe that she is now so determinedly pursuing; Nicola Sturgeon is a leader who declared that the last Scottish independence referendum was a “once in a generation” event and yet who now wishes to revisit the issue, although in much changed circumstances.

Aside from the arguments in favour of both unions, and aside from base political calculations about timings and tactics, should the Westminster parliament attempt to stymie the proposed Scottish vote? No, has to be the answer, for there is no “statute of limitation” on the frequency of any referendums, and the soon-to-alter constitutional position of the UK is an obviously fundamental change that has to be acted upon by the Scottish government. If the Scottish people, in this case through their parliament and government, desire a further vote, than they should not be deprived of that opportunity. Ms Sturgeon is right to propose holding the vote after the broad terms of Brexit are known, but before any new treaties are signed and exit is finally agreed. That is precisely the right time, and, indeed, it would also be precisely the right time for the UK as a whole to have its say on the terms of exit, a democratic imperative that will become clearer as Britain heads towards the exit and reality dawns upon our leaders and voters. In late 2018 or early 2019, a “joint” referendum could be held across the UK, with an additional question on the Scottish ballot paper concerning independence for Scotland. The possibility of Scotland leaving the UK in the event of a “leave” vote in that referendum would also influence English, Welsh and Northern Irish voters. It would be a momentous day for democracy, and one that might actually deliver that elusive reward promised by the proponents of such plebiscites – that it would settle these issues for good – although not symmetrically. For if the UK leaves the EU, and Scotland leaves the UK, then there could be no going back, at least for many years.

As to the substance of the issues, there are compelling analogies with the approaching split between the UK and Europe. The rest of the UK is Scotland’s largest trading partner – some 70 per cent of Scottish “exports”, and the cultural, family, economic, financial and other links between the component parts of the UK are obviously even closer than they are between Britain and the rest of the European Union. There are many more things in common, not least language, though English was often violently imposed and only now are guarantees for other languages being introduced. So the disruption would be considerable, and, on current trends, Scotland might well be worse off in the short run, though with the prospect of being able to order its own affairs in future to deliver greater prosperity, especially if Scotland remains in the EU and the rest of the UK leaves. Whether Scotland would have to adopt the (European) single currency remains an open question, as does the use of the British single currency (the pound sterling), so disputed at the time of the last vote.