For the most of the referendum campaign, the picture painted by the polls has been one of relative stability. The results produced by a company in one month have often been much the same as those it had reported the previous month – and were often much the same as those that eventually appeared the next month. As our poll of polls illustrates, up to now the only period when there has been a consistent movement in one direction was during the winter of this year, when the Yes side added some four points or so to its tally.

Against that backdrop today’s poll from YouGov – for both The Sun and The Times – will come as both as a surprise and (for the No side at least) a shock. It puts Yes on 42%, up four points on YouGov’s previous reading just a fortnight ago (after the first leaders’ debate) and No on 48% (down 3). Once the Don’t Knows are excluded Yes are on 47%, up four points, by far and away a record Yes vote from a pollster that has hitherto tended to paint a relatively pessimistic picture for the Yes side.

Indeed, this is the second time in a row that YouGov have reported a record high Yes vote. In contrast, in its last poll but one, conducted as recently as the beginning of August, the company put Yes on just 39% (after Don’t Knows had been excluded), as indeed it did too in its poll before that, conducted at the back end of June. In short today’s poll suggests that Yes support has increased in a month by as much as eight points on what was a seemingly stable baseline.

At minimum today’s poll strengthens the impression already created by a number of recent polls that the No lead may have narrowed somewhat in recent weeks. Our poll of polls now stands at Yes 45%, No 55%, equalling the all-time high Yes vote previously recorded in the second half on April. However, whereas on that occasion the high Yes vote could be regarded as the result of the accidental predominance of polls from pollsters that always tended to paint a relatively optimistic picture for the Yes side, this time around that is less obviously the case.

Indeed we might remember that just three moths ago YouGov’s Peter Kellner was arguing that the estimated Yes vote in polls conducted by Survation was too high. Now YouGov put the Yes vote just as high as Survation do.

Of course, too much should never be read into one poll – and we still await a poll that actually puts the Yes side ahead. But there seems little doubt that this poll will electrify the campaign. The Yes side will now be able to argue with some conviction that they have gained momentum and can hope that they might yet secure a dramatic and historic victory. The No side, meanwhile, will be asking itself why it now apparently finds itself in a desperate last minute fight to keep Scotland in the Union.

Today’s poll offers some clues. As we have repeatedly emphasised, the issue that above all seems capable of persuading people to vote Yes or No is whether they think independence would be good or bad for Scotland’s economy. Up to and including its poll in June YouGov had never found more than 30% saying that Scotland would be economically better off under independence. Now, having risen to 32% in YouGov’s poll a fortnight ago, that figure now stands at 35%. At 44% the proportion who think Scotland would be worse off still outnumber the optimists, but they are fewer in number now than in any previous YouGov poll. It looks as though the Yes side has made some significant ground on the crucial economic debate in recent weeks.

Second, although unfortunately in this poll YouGov asked new questions about people’s attitudes towards the currency issue, and thus we cannot see whether attitudes have changed in the wake of Mr Salmond’s clarification of his stance in the second leaders’ debate, it certainly looks as though this issue is still not scoring for the No side in the way that it believed it would. It remains the case that most voters – including most No voters – would like an independent Scotland to keep the pound, while many voters (or Yes supporters at least) still believe that this is what would happen.

As many as 56% of all voters – including 54% of No voters – would like an independent Scotland to keep the pound as part of a monetary union with the rest of the UK. Meanwhile 67% believe that an independent Scotland would carry on using the pound, either as part of a monetary union (41%) or otherwise (26%). Amongst Yes voters that last figure stands at no less than 87% (including 67% who think there would still be a monetary union, but even just over half of No voters (54%) reckon Scotland would keep the pound. In short the No side’s claim that an independent Scotland would not be able to keep the pound is still widely disbelieved and goes against the grain of what many of their own supporters would want to happen in the event of a Yes vote.

Third, the Yes side appeared to have secured some traction in its recent claim that the NHS might suffer if Scotland were to remain in the Union. As many as 42% think the NHS will get worse if Scotland remains in the Union, while only 9% believe that it will get better. In contrast as many as 37% believe that the NHS would get better under independence, while only 29% believe it would be worse. Of course many of those who think the NHS will get worse if Scotland remains in the Union are existing Yes voters (while contrary to what the Yes side themselves have been claiming, fewer women than men supports its view about which option offers the brighter future for the NHS). But at least the Yes side have identified a supposed risk that at least their own supporters find credible – in sharp contrast to position in which the No side finds itself on the currency issue.

Fourth, the Yes side appear to have made particular progress amongst the less well-off C2DE social groups, at whom much of its campaigning has been targeted in recent weeks. Support for independence amongst such voters is up nine points on a month ago (after Don’t Knows are excluded) whereas amongst more affluent ABC1 voters the swing has been a more modest six points. Doubtless this helps explains why support for Yes amongst those who voted Labour in 2011 has increased over the same period from 18% to 30%, though in truth the former figure was always rather low as compared with the findings of most other polls.

Just one group of voters appear to have resisted the tide towards a higher Yes vote – older voters. Despite the particular importance of the NHS for such voters, at 31% support for Yes amongst the over 60s is actually slightly down (by two points) on a month ago. Indeed, but for the Yes side’s weakness amongst this group YouGov would be putting Yes in the lead. Will pensions now be one of the crucial battlegrounds in what promises to be a very keenly contested last two weeks?