Since my last post I've had a chance to look at the YouGov datasets, and I was particularly interested in seeing what has happened to SNP voters from 2015. It's important to stress there has been movement in both directions - 10% of people who voted Labour two years ago are now planning to vote SNP, as are 7% of people who voted Liberal Democrat, and even 1% of people who voted Tory. But obviously that is more than offset by the people who have moved from the SNP to a unionist party.
It shouldn't be any great surprise that 10% of the SNP's support has moved direct to the Tories - there are bound to be voters, especially in rural areas, who used to vote SNP for reasons that had nothing to do with the constitution, and who now feel that a stridently pro-Brexit/anti-independence party better reflects their views. More interesting, though, are the 8% of SNP voters who have switched back to Labour. Because the SNP's vote was twice as big as Labour's in 2015, that means (if the poll is accurate) there has actually been net movement from the SNP to Labour, in spite of the fact that Labour's overall vote has continued to fall. I'm not convinced that finding can be explained by people having a change of heart on independence, because Labour's current coalition of support is considerably less anti-independence in character than its 2015 coalition was. 25% of people who currently plan to vote Labour would vote Yes to independence, compared to just 13% of people who voted Labour in 2015. So it looks very much like there is a significant number of people out there who are pro-independence, and who have actually voted SNP at least once in the recent past, but who are nevertheless planning for some inexplicable reason to vote for the sinking ship that is Labour. If the SNP are looking to recover some lost ground, that group may be the most obvious low-hanging fruit.
It's smaller beer, but we can also take some heart from the fact that 2% of SNP voters from 2015 say they plan to vote Green. With no Green candidate to vote for in the vast majority of constituencies, it's not unreasonable to suspect that most of those votes will be heading back to the SNP - which could be enough to boost the overall SNP vote share by 1%.