Recovery risk for government borrowing
Whenever I find myself in any gathering of bankers, business people or politicians, the question they ask more than any other is whether the government will be able to borrow all it needs from markets - or whether at some point big investors will lose their appetite for gilt-edged stock, the exchequer's IOUs.
A good person to tap on this - though one who is not without a vested interest - is the Treasury's banker, Robert Stheeman, the head of the Debt Management Office, whose job is to raise all those hundreds of billions of pounds to cover the gap between tax revenue and public expenditure.
This year he has to sell an utterly unprecedented £220bn of gilts. That's more than four times the finance that he's typically had to find in recent years. And it would expand the existing stock of gilts - or the size of the market - by more than a third.
And, what's more, with the government refusing to countenance significant spending cuts or tax rises, he'll have to raise a similar amount next year too (even if a new government were to massively reduce public expenditure, there would be a lag before there was an impact on borrowing).
So - I asked him, in an interview for the Today programme - how great is the risk that investors will sit on their hands? Does he lie awake at night fearing a re-run of the 1970's, when a Labour government had to be bailed out with emergency financial support from the International Monetary Fund?
Well, Stheeman doesn't have bags under his eyes and sees a funding crisis as a very remote danger - largely because the market for gilts is much wider and deeper than it was.
For example, it has become much more international, with a record 36% of gilts now held overseas.
Now, you might point out, he would say that, wouldn't he?
That said, he was much less dogmatic about whether the government might end up having to pay a much higher interest rate to borrow - which is hugely important, because an increase of one percentage in the cost of borrowing £200bn would be £2bn that wouldn't be available to spend on public services every single year till the debt is repaid.
If, for example, the UK lost its impeccable AAA debt rating, that would almost certainly push up funding costs - not least because some of those helpful overseas buyers are central banks which aren't permitted to hold sovereign debt rated at less than AAA (though it was striking that Stheeman told me that he didn't think a downgrade of just a notch would make it significantly harder for him to raise what he needs).
But here's one reason why it's so difficult to judge where the cost of borrowing for the government will settle in the coming few months: the Debt Management Office has sold fewer gilts than have been bought by a separate part of the public sector, the Bank of England.
In April, May and June, Stheeman and his team flogged £57.9bn of gilts, while Mervyn King's traders waded into the market to buy £77.7bn of UK government debt.
The Bank of England is buying as part of its so-called Quantitative Easing programme to increase the stock of money in the economy and cut the cost of credit.
But the Bank has almost disbursed the £125bn allocated in total to the scheme - and was somewhat equivocal a couple of weeks ago about whether it will increase its gilt-purchasing budget.
Investors seem persuaded that the Bank of England will buy a bit more - although we'll have the first test in a gilt auction this morning of whether the Bank's equivocation is seriously unsettling investors.
Gilt prices have been falling after the large penny dropped in markets that there may not be many more weeks before the Bank of England transmogrifies from a massive net buyer of gilts into a potential seller.
Where the gilt price settles then is - as Stheeman implied - not something that can be predicted with scientific certainty.
And here's the great and painful paradox.
If the Bank of England stops buying at a moment when investors become a bit more confident about prospects for the global economy and our economy - if they become less averse to risk and more interested in buying assets other than AAA sovereign debt - well, then it might become altogether more tricky and expensive for the government to borrow.