Conservatives gathering in Manchester for their annual Conference were jubilant. They believed that they have Labour on the run, intellectually, politically and – what seems even more important to them – electorally. Two weeks ago a look at the earliest evidence suggested that they might be right, and that Labour was failing to enjoy any sort of uniform national “bounce” at all from the election of Jeremy Corbyn as their new leader. What new votes they were attracting – mainly from the Greens in urban areas – would do them little good in winning any extra seats; the geographical spread of any uplift, for instance concentrated in Scotland, was similarly pointless in returning more Members of Parliament

Well, now we have a little more evidence – two more weeks of by-elections, three UK-wide opinion polls and a properly-weighted voting intentions poll from Wales. And the picture looks very, very similar – indeed, so suggestive across the board that we can start to draw out some general trends and lessons.

We can start with this: there is indeed now a national Corbyn “bounce”, but the effect is weak if we look at the statistics in historical perspective. There have now been seven nationwide polls since Corbyn became leader, two of them by one company (ComRes) but using different methodologies, and a further two both by YouGov. If we compare like with like, while averaging the results of the two YouGov polls, the mean level of Labour’s vote increase across all pollsters is 1.5 per cent. Since the Conservatives’ polling position seems also to have softened just a little, the gap between Labour and the Conservatives has shrunk by an average of 2.3 per cent.

So, Labour has made some progress – but on a very small scale, if we examine past trends. If we look at the other five Labour leaders to have taken over in Opposition during the modern era (Foot, Kinnock, Smith, Blair and Miliband), their average ‘bounce’ on a comparable basis was 4.1 per cent. Foot put on 2.3 per cent, Kinnock 7 per cent, Smith 4.5 per cent, Blair 4.9 per cent and Miliband two per cent. What progress Corbyn has made is clearly some way short of those numbers: he even does worse than Foot and Miliband’s polling debuts.

It is, to be fair, a little early to conclude that the Corbyn effect is as puny as this suggests. Matt Singh’s work for The Times suggests that the positive effect of any new leader peaks at around the three month mark, before declining gradually thereafter. So we need to come back and revisit this question in December, when on past evidence we would expect Labour to have put on about five per cent. If they haven’t, or if they have indeed stood still or gone backwards, our initial foreboding based on these numbers will have been borne out. For now, the weakest polling boost in the recent history of Labour’s new Opposition leaders is something to record in itself.

And where are those rather small numbers of extra voters being attracted? Here there is much more to go on. A YouGov/ ITV/ Cardiff University poll that came in last week showed quite a substantial Corbyn “bounce” in Wales – of five per cent in terms of Westminster voting intention, while the Conservatives dipped by two per cent. Recent council by-elections in Scotland have also suggested a Labour recovery in that country, though in this case from a very low base indeed. Last week there were seven of these contests, of which six saw the SNP and Labour both run candidates. These allow us to check whether the six to seven per cent or so swings in Leith, Midlothian and Ayr East in the previous three Scottish contests were outliers, as those results were much better for Labour than some of the huge swings that party was hit by over the summer. And, indeed, the swings here were of about the same magnitude: a 7.6 per cent swing on the last time these seats were contested in 2012. That implies that Labour might be just over 15 per cent behind the SNP nationally – rather better than they were doing in the last actual all-Scotland poll we have, in which they trailed by 30 per cent (51 per cent to 21 per cent) back in July. Since Labour was 26 per cent behind the SNP in the General Election, this represents perhaps a 5.5 per cent swing back to Corbyn’s party since May. There do seem to be some signs of life in Scottish Labour yet.

There is, however, one enormous problem with this (very welcome) progress: it would not gain Labour many more Westminster MPs. A 3.5 per cent swing in Wales would net Labour just three more seats (Gower, Vale of Clwyd and Cardiff North) – two of which they actually managed to lose, by tiny margins, to the Conservatives back in May. Their next target (Preseli Pembrokeshire, which they would gain on a swing of just over six per cent) would seem, given these results, entirely out of their reach. Similarly, a 5.5 per cent of so swing from the SNP to Labour in Scotland would see Labour take back just two of the forty seats they lost in 2015: East Renfrewshire and Edinburgh North and Leith. These gains may also be even more paltry than this suggests. If the Conservatives press ahead with reducing the size of the House of Commons from 650 to 600 (by no means certain at this point), Wales will lose ten MPs, and Scotland seven. On a pro rata basis, that would reduce Labour’s gains in Wales to two. Even if they did manage to win those two extra seats in Scotland on the new boundaries (by no means certain) that would give Labour just four more seats.

The same is true of Labour’s advance in those seats where they can squeeze the Greens. The average decline of the Green vote in English council by-elections since Corbyn’s election as Labour leader (and where they have stood) has been 4.6 per cent of the vote, or about 38 per cent of the Green vote since the last time these seats were contested during the last Parliament. Suppose – very unrealistically – that all of that share of the Greens’ general election total (swollen from the levels it stood at when most of these seats were contested) went to Labour in England. That would see just six more Conservative seats move over to Corbyn’s party on the back of ex-Green votes, which might be pretty much where Labour’s English advance stalls. The need for gains from the Greens is put into stark contrast by the overall scale of Labour’s upwards push. In all the English by-elections since Corbyn’s election taken as a whole, Labour have moved forward only 0.9 per cent - a tiny gain that would mean they depended almost entirely on these Green defectors if they are to win more Westminster seats.

So there we have it. Labour is apparently moving forward in Wales, enjoying some incremental gains in Scotland, and is able to profit where there is a substantial Green vote to attack. All of that – even during a new leader’s honeymoon, and even on the best possible assumptions for the party – might net it ten or eleven seats, two of them from the SNP. The Conservatives would lose their overall majority, but only barely: a highly unlikely outcome given the scale of the gains they must feel are theirs for the taking in England.

So what can we conclude about Labour’s electability under Corbyn? We should all be cautious before reaching any firm conclusions. The data is still fragmented, patchy and scarce. But from what we do know, and what we can already see, only one judgement is possible: Labour has somehow found the best possible way to concentrate votes and strength where it cannot use them or does not need them, and give ground where it desperately needs to make it up. If it continues on the path it has set itself, the very early evidence suggests that Labour is going to be mauled in 2020, perhaps very badly. The party has conspired to locate precisely what might be called its bitter spot. This equates to the exact place where they make the least possible gain of MPs for votes: the opposite of that sweet spot the Conservatives managed to find in 2015, converting ballots into seats in a more efficient way than they have managed for decades. It is a bitterness Labour is probably going to have to get used to.