Have you sometimes wondered how many wheels have to come off a bus before it crashes?

In The Italian Job, I recall it took only one wheel to come off in order for the bus to be left teetering on the brink of a precipice with the gold literally slipping from the grasp of Michael Caine and his team.

Not so in SNP La La Land, where all four wheels seem to be able to come off their bus of economic plans for a separate Scotland but it just keeps skidding on down the road, sparks flying.

Wheel one: liabilities. This has always been there but thanks to Ian Blackford, the SNP’s smoothie at Westminster, it has floated back up to the surface again with his ludicrous claim that the remainder of the UK would continue to pay the pensions of Scottish pensioners if Scotland were a separate country. Does he know this will not happen or, perhaps more worryingly, does he actually believe it? There is no UK pension fund, your payment of national insurance may entitle you to a pension in the future but your contributions actually pay your parents’ pension and you hope that your children’s contributions will pay yours. A separate Scotland would have to pay its own pensions.

This wishful thinking extends to debt too. UK sovereign debt is now nearly £2,500 billion and Scotland has received more than its 8% population share of the spending by the UK which has caused that deficit. To think that in some way Scotland could leave the UK and not take its share of the UK debt is for the birds. On the basis of population share, our allocation would be around £200bn.

Wheel two: currency. The SNP’s position on the currency a separate Scotland would use is now just excruciatingly embarrassing.

Their previous bold assertion we could keep using the pound was never going to stick and they now say Scotland would establish its own currency. Unfortunately, this isn’t a viable policy either and not just because as a new applicant to the European Union, we would have to sign up to using the euro. The other unmentionable issues are the value of the new currency and the interest rates we would pay. This isn’t just some theoretical problem – Scottish workers would be paid wages in a currency which is likely to devalue and would be paying interest at a higher rate on their mortgages.

If Scotland had £200bn of debt and we make the optimistic assumption that the interest rates for a Scottish currency were only 1% higher than for sterling the extra cost is £2bn – an eighth of the Scottish NHS budget.

Wheel three: the budget deficit. Scotland does generate about its population share of total UK gross domestic product, around £170bn in 2019 before Covid hit. However, public expenditure per head in Scotland is considerably higher than in the UK as a whole. The result was a Scottish budget deficit of over 7% of GDP in 2019 compared with under 2% for the UK . Fighting Covid drove Scotland’s budget deficit last year to over 20% of GDP. The level Scotland would need to aim for in order to re-join the EU is under 3%. Even eliminating the 4% pre-Covid difference would cost around £7bn. How is a gap equivalent to over 40% of NHS spending in Scotland going to be filled – is it savage spending cuts or massive tax rises?

Wheel four: the UK single market. Of course, Scotland has important trading links with the EU and the rest of the world but these are dwarfed by its trade with the rest of the UK. If a separate Scotland lost frictionless trading access to the rest of the UK – which would be inevitable if Scotland joined the EU – it would be disastrous for Scottish businesses and employment. The nationalists would have us believe that Brexit makes separation more desirable – the reality is that it means breaking up the UK would be an economic catastrophe for Scotland.

All the wheels are off the nationalist bus, but the diehards press on.

Actually, quietly, I think the Scottish electorate has rumbled them and the ditch is as far as they will get.

Guy Stenhouse is a Scottish financial sector veteran who wrote formerly as Pinstripe