That sound you just heard was the entire Scottish Labour Party — all 12 of them — slapping their foreheads in frustrated unison. In an interview with Iain Dale at the Edinburgh Fringe, John McDonnell confirmed that Labour would not stand in the way of the SNP holding a second referendum on Scottish independence. The Shadow Chancellor told the LBC host:

“ ‘We would not block something like that. We would let the Scottish people decide. They will take a view about whether they want another referendum. Nicola Sturgeon said by late next year or the beginning of 2021. We would not block something like that. We would let the Scottish people decide. That’s democracy. There are other views within the party but that’s our view.’

This is the polar opposite of the message Scottish Labour has been trying to get across to the voters. The party’s perceived ambivalence on the Union was credited with helping Ruth Davidson replace the Caledonian comrades as the official opposition at Holyrood in 2016. Earlier this year, when Labour’s Scottish leader Richard Leonard was asked about a second referendum, he said ‘there is no case for one’ and, pushed further, insisted there was no mandate for the SNP to hold one. Around the same time, Leonard lamented that Scottish Labour had been seen as ‘bogged down too much on constitutional issues’ and said the party was ‘determined not to repeat that mistake’.

McDonnell’s intervention has dragged them back into the bog. Their professed opposition to Indyref2 won’t count for much since the voters know these decisions are taken in the leader’s office. McDonnell added: ‘The Scottish Parliament will come to a considered view on that and they will submit that to the government and the English Parliament itself. If the Scottish people decide they want a referendum that’s for them.’

Every so often, Jeremy Corbyn pops up to throw the SNP a bone, much to the horror of his Scottish foot soldiers, who know how toxic the independence issue is with their voters. Whether this is a calculated appeal to the public sector leftists who abandoned Scottish Labour for the SNP a decade ago or is, as Francis Urquhart liked to say, ‘putting a bit of stick about’, is anyone’s guess but I suspect it’s a bit of both. (Stalinists are even keener on party discipline than Tory chief whips.) The chance that McDonnell doesn’t know what a juicy chunk of red meat his calling Westminster ‘the English Parliament’ will be for Scottish Nationalists is somewhere between zero and a large number with a minus in front of it.

This strategy — if that is what it is — was already in full swing before the 2017 election and still Labour managed to pick up only half the number of Scottish seats the Tories did. Maybe it stands a better chance in an election with a straight choice between Labour and Polo Mallet of the Scots, Boris Johnson. But coming, as McDonnell’s comments do, the day after a Lord Ashcroft survey showing a majority of Scots (and 40 per cent of Scottish Labour voters) are in favour of independence, it could just as easily appear to voters that Labour has taken fright and changed its constitutional stance over a single opinion poll.

McDonnell’s comments are certainly ammunition for the SNP but it is Ruth Davidson and Jo Swinson who will be the happiest tonight. In a few sentences, the Shadow Chancellor has undone years of work by Holyrood Labour and may well have sliced Scotland’s Unionist troika into a Tory-Liberal duopoly.