Numbers - or, rather, our inability to agree precise numbers to set against the challenges facing all governments - have their own lethal way of blowing up in politicians' faces. In recent days, radical upward revisions to the number of migrants coming to Britain and the number of new jobs they have managed to fill in our economy have left squirming London cabinet ministers issuing clarifications and grovelling apologies. And the escalating billions being shelled out by the Bank of England to underwrite the ailing mortgage bank, Northern Rock, raises the spectre of what was once the pride of Newcastle becoming the first (forced) nationalisation in Britain for decades.

In Scotland, the latest population projections raise profound questions about how robust policy- making can ever be if we don't know, with any kind of clarity, how many of us will be around in future. Five years ago we were being told Scotland's population would fall below five million by 2009. Now it is projected to grow to a new all-time high of 5.37 million by 2031, before shrinking. It won't dip back below five million, government demographers now opine, until 2076. But, hey, what's a variation of 67 years - more than average male life expectancy in some parts of Glasgow - between friends?

If we are to believe a welter of overwrought heartsearching which has erupted across all sections of the media down south, one particularly contentious set of numbers even has the megatonne potential to blow the existing British constitution to smithereens. It is the balance sheet of who puts what into - and who gets what out of - the existing Union. Yet again we Scots are being fingered as unreformed "subsidy junkies", taking much more out of the British state than we put in.

From "Jocks Away" in southern editions of the Daily Mail last Saturday, suggesting Scotland's entire political class has been "infantilised" by not having to raise the public money it is allocated by the Treasury before it spends it, to a leader in this weekend's Economist suggesting it would be no bad thing if Scotland - "a country already over-administered, over-represented and over-financed" - was allowed to tax itself, we are back in the midst of what the writer George Rosie once called "an extraordinary sneerfest about our shiny new Scotland".

Rosie's original riposte, in his Scotching the Myth film for Scottish Television in 1990, was to point out that the most feather-bedded and tax-cosseted part of the UK at that time was actually London and the south-east, vacuuming up most of the principal institutions of the British state, enjoying the lion's share of the 15% of annual public expenditure in the UK which cannot be identified as being spent in any particular constituent region or nation, and commanding infrastucture projects with price-tags that dwarfed what was being built elsewhere across Britain.

He revisited his myth in 1995 and again in 2000, whenever sections of the metropolitan commentariat convulsed anew over Scotland's "pork-barrel preference" or a Gaelic culture dominated by "besandalled and bearded former polytechnic lecturers". Now, with a minority SNP government actually in power at Holyrood, it appears to be open season yet again on one part of a devolving United Kingdom where, we are told, the politicians' sole skill is "extorting money from Whitehall and justifying outrageous budgets".

Elsewhere in The Herald today, my colleague David Leask has updated aspects of George Rosie's riposte by setting out, with some help from analysis done by consultants Oxford Economics, five of the more enduring myths about where Scotland really stands in relation to the other nations and regions that make up the United Kingdom in terms of who gets what and who provides what within our present constitutional settlement.

The picture that emerges is much more nuanced than the black-and-white certainties of some London-based tabloids. Scotland is not some subsidy-obsessed outrider of the British state, offering little in return by way of tax revenues. There are other parts of the UK which receive higher per capita allocations of public expenditure than Scotland.

Most English regions contribute less, per capita, in tax revenues. If all revenues from North Sea oil and gas production are allocated to Scotland's account (an assumption even most Nationalists might judge unjustifiable) it is possible, in a good year, to come close to balancing the Scottish budget. Arguably only London does better.

But when George Rosie first tried to Scotch his Myth, London was seeing inordinate amounts of public money poured into the regeneration of Docklands. The new British Library in St Pancras was bursting its construction budget in ways Edinburgh's parliament building would, later, come no where near matching. Then there was the Dome. Now there is Crossrail and the 2012 Olympics.

That's what happens in capital cities with the kind of global clout London undoubtedly enjoys. And the unequal ebbs and flows of public expenditure and tax revenue streams across the rest of these islands is also what happens all over the world. It is the inevitable consequence of every government's efforts to raise more money from those who can afford it and spend more on where the need is greatest.

It's an imperfect art. But when these imperfections become characterised as some parts of the polity playing by completely different rules, misunderstandings can quickly fester into something much more serious. I've no doubt Alex Salmond will be delighted with the Economist's call for Scotland to be allowed to tax itself. And right at the top of his shopping list will be his desire to control the tax take from the oil and gas fields off Scotland's shoreline.

Our First Minister makes no secret of his desire to see Scotland independent of the present Union. But he knows better than anyone just how big a challenge he still faces turning a wafer-thin hold on devolved power at Holyrood into a clear mandate to negotiate Scotland's departure from the UK. The findings of this week's Scottish Social Attitudes Survey will not have surprised him. Many of those who embraced the invitation to vote for Alex Salmond as First Minister in May were doing exactly that.

They wanted to see whether he would fight for Scotland's interests more effectively than his predecessors. They'll need clear evidence that he can before they'll agree to join him on later stages of the constitutional journey he has in mind for them. But he might yet be helped in the business of persuading them, if some of our friends in the south persist in painting a picture of the Scots that wilfully distorts tax and spend reality.