IF the SNP’s mammoth lead of 52 per cent to 24 per cent in the new TNS poll seems eerily familiar, there’s a good reason for that.

The showing for the two main parties is identical to an Ipsos-Mori poll conducted in January, and almost identical to the famous Ipsos-Mori poll in October that first announced the SNP surge.

So is this another steady-as-she-goes poll? Far from it. Until now, Ipsos-Mori have been ploughing a lonely furrow as the only firm to have produced SNP leads in the high 20s, or to have reported an SNP vote of above 50 per cent.

The latter finding, in particular, always seemed intuitively implausible, and for as long as other pollsters were unanimous in offering more conservative numbers, it was easy to dismiss.

Suddenly we now have a second firm suggesting that an absolute majority of the Scottish electorate plan to vote for Nicola Sturgeon’s party next month, and the possibility that such an extraordinary result might just be on the cards will have to be taken a little more seriously.

The broader significance of the new poll lies in the trend. Since the latter half of October, any changes in voting intention reported by pollsters have generally been of a relatively trivial nature, and have looked very much like normal fluctuations caused by the standard three per cent margin-of-error.

The first sign that the state of play might finally be budging came in last week’s YouGov poll, which suggested a substantial increase in the SNP lead since the autumn – a change that was difficult, albeit not quite impossible, to explain away as the effect of normal sampling variation.

In the case of TNS, the swing to the SNP recorded since the first post-referendum poll from the firm is so enormous that there is no credible room for doubt that it reflects genuine movement on the ground.

It would be easy to assume that TNS and YouGov are straightforwardly corroborating each other’s findings, but there’s a complicating factor. The pattern shown by YouGov suggests that the boost for the SNP occurred fairly recently, and was possibly caused either by the fallout from the “Frenchgate” affair or by Sturgeon’s much-praised performance in the UK-wide leaders’ debate.

In contrast, the fieldwork for the TNS poll stretched over a much longer period, and dates back further even than the last-but-one YouGov poll, which produced a much more familiar result. It looks, therefore, as if TNS picked up a swing significantly earlier.

This slight contradiction should caution us that we can’t necessarily expect all pollsters to replicate the same trend. TNS are unique in that they still collect their data in the old-fashioned way, via face-to-face interviews.

It may be that going out into the ‘real world’ to find a completely fresh sample for every poll will help them detect big changes in public opinion that aren’t always picked up by internet firms, who are reliant on volunteer polling panels that probably contain a disproportionate number of politically committed people.

During the independence referendum, two of the three pollsters that reported a huge swing to Yes in the closing stages of the campaign used a ‘real world’ methodology (either face-to-face or telephone), while most online polls showed only very modest changes.

Other than TNS, there is only one firm that has not relied to date on an online panel for its full-scale Scottish general election polling – and that firm is Ipsos-Mori.

It will be fascinating to see whether the next Ipsos-Mori poll follows the example of TNS by suggesting a further sizeable boost for the SNP.