RING Ring. Hello, it’s your line to David Davis. I’m afraid all of our Brexit ministers are exceptionally busy right now, but your call is very important to us so please listen carefully to the following message: “Brexit means Brexit means Brexit ... ”

The idea of the Scottish Government having a “direct line” to the Brexit Secretary, Mr Davis, at least demonstrates that Theresa May has a sense of humour. As the most militant anti-European right winger in the cabinet, the former SAS reservist is not perhaps the most sympathetic ear to put on the constitutional hotline.

There is an ugly mood in England over Brexit as the lack of any plan becomes apparent and the UK Government resorts to the lowest common denominator of immigration. But the ugliness is also apparent in antagonism towards Scotland securing any kind of special arrangements with the EU. I have been on a number of London-based phone-in programmes recently during which it has become clear that this is regarded by UK politicians and commentators as a form of treachery; an attempt to “reverse Brexit by the backdoor”.

Read more: Nicola Sturgeon - I won't allow Scotland to be driven over a hard Brexit cliff edge

The City of London having a special arrangement seems to be unobjectionable as that would be for “sound economic reasons”, whereas Scotland is not an area where, it seems, economic rationality applies. Brexit must be a one for all, a united “UK-wide” solution, except for the car industry, bankers, Northern Ireland and so on.

Yet, Scotland’s economy requires up to 100,000 migrants each year, according to the demography expert Professor Robert Wright. That’s not a figure the Scottish Government recognises, though during the referendum it was widely accepted that Scotland needed at last 24,000 migrants a year to keep the working age population stable.

The reason is simple: Scotland has an ageing population (the proportion of over 75 year olds is expected to rise by 80 per cent in the next 20 years) and dwindling numbers of working age, tax-paying employees. How do you afford all that elderly care, as well as other public services, when tax revenues are on the slide?

Read more: Nicola Sturgeon - I won't allow Scotland to be driven over a hard Brexit cliff edge

In countries such as Canada and Australia this is addressed by a visa system that exempts regions with low populations from overall migration limits. This was the system that the leading Brexiter and former Lord Chancellor Michael Gove said could apply to Scotland after Brexit. But Mrs May seems to regard the idea of ceding control of immigration to Scotland as anathema; though, again, such a visa scheme is reportedly being considered for the City of London to deal with skill shortages.

The fear in Westminster is not so much of a soft Brexit as a Swiss-cheese Brexit: full of holes where different sectors of industry and regions have special deals; hence the hard line at yesterday’s Downing Street dialogue of the deaf. But this is something Brexiters might have thought about before now.

The UK is a multinational state with devolution of legislative authority. You can’t just behave as if the UK were the old monolithic unit it was in the 20th century.

The London-based Institute for Government has rightly pointed out that the Prime Minister risks a constitutional crisis if the UK Government fails to recognise this. The Scottish Parliament may be constitutionally subordinate to Westminster but it is supposed to give its consent to any UK legislation that impinges on its powers. The 2016 Scotland Act placed this Sewel convention on a statutory basis, making it a legal requirement.

Read more: Nicola Sturgeon - I won't allow Scotland to be driven over a hard Brexit cliff edge

If Westminster simply overrides this it means, effectively, that the devolution settlement no longer stands. Scotland’s Parliament is effectively diminished and made a “creature of Westminster”, a regional unit of local government. Scots might be prepared to accept that but they might not.

The deputy leader of the Labour Party, Alex Rowley, has called for a constitutional convention to defend the Scottish Parliament and the other devolved parliaments against Brexit centralisation. This is an interesting idea and has been supported by former SNP minister Kenny MacAskill. The 1988 Scottish Constitutional Convention included a declaration of sovereignty, signed by Scotland’s MPs. It asserted “The sovereign right of the Scottish people to determine the form of government best suited to their needs”.

That looks like a declaration that every MSP in Holyrood should be able to reaffirm. It might be worth dusting it down and convening a 21st century convention if only to unite Scotland’s civic society behind the First Minister as she tries to get her message through the fog of Brexit. Hello – is anyone there?