This week marked something of a watershed moment, certainly for the Salmond/Sturgeon scandal and perhaps yet for devolution too. After months of watching the Scottish Government hamstring the official Holyrood inquiry into its mishandling of allegations against Alex Salmond, Westminster stepped in.

Liam Fox prepared the way, quizzing the Prime Minister and then the Speaker about Parliament’s responsibility towards civil servants in Scotland. As part of the broader Home Civil Service, it follows that they are accountable to – and should be able to call on the support of – their most senior colleagues in London. And that there is a Minister somewhere answerable to Parliament about them.

Then David Davis stepped up, using parliamentary privilege to put into the public domain what he claimed was fresh evidence he had received from a whistle blower about the conduct of the Scottish Government – evidence his source apparently claims ‘point to collusion, perjury, up to criminal conspiracy’. I wrote up his speech in detail yesterday.

Both interventions reflect a potential new front opening up in the constitutional battle between the Union and its opponents. Where the UK Internal Market Act has seen the Government start to assert itself more in previously devolved spheres, this represents the legislature stepping up to its responsibilities.

Unionists won’t want to rock the boat too much ahead of May’s Scottish elections. But when those are over, there is an interesting debate to be had about procedural or institutional changes that could address the issues these interventions have raised – perhaps bundling privilege for MSPs with stronger lines of accountability for civil servants, or even reviving Fox’s old idea of drawing an upper house for the Scottish Parliament out of the House of Lords. I’ll be looking at this in more detail later.

In the meantime, the polls continue to show a slide in the SNP’s support. They’re still on track to hold on to power in Edinburgh, but it looks as if whatever magic insulated their public standing from the torrent of bad stories their government was generating may finally be running out of steam. When you see Nationalist ministers snarling about ‘rigged polls’, and SNP efforts to put ‘indyref2’ on the ballot paper, you know they’re spooked. The latter play is especially important because it suggests they may be falling back on their (very substantial) core vote.

There have also been a string of polls from different companies putting the Union ahead in a hypothetical second referendum. Obviously this no more represents the ‘settled will’ of the Scots than did the previous string of pro-separation results, but it will all make it easier for the Prime Minister to maintain his opposition towards a granting a re-run of 2014.

One poll, commissioned by campaign group Scotland in Union, used the EU referendums Leave/Remain framing and delivered a pro-UK share of 57 per cent – a remarkable illustration of how high the stakes would be in negotiating the question and how reckless was David Cameron to concede so much to the SNP ahead of the 2014 vote.

The story continues to develop: just two days ago, after pressure from the Conservatives, the Scottish Government have finally published their own review into the Salmond fiasco, which calls for the process to be taken out of the hands of civil servants.There will doubtless be more to come in the weeks ahead.

Unionists will struggle to oust Sturgeon as First Minister. But she has already served in that role for seven years, and is finally showing signs of political mortality. If they can do enough to let Boris Johnson kick independence into the long grass, that might very well be enough.