Murray often claims to be reporting something he was told by an insider at the Foreign Office.

I'm not sure whether in general to believe him.

In this case I'm convinced, from what we know now, that he was telling the truth: that the government was planning, at the time the backstop was negotiated, for how to break it, and that this was unusual enough for Foreign Office legal advisers to be distinctly unhappy about it.

I don't think this demonstrates that every time he claims he's reporting insider information he's telling the truth. But it does weigh in the direction of treating him as a truth-teller.

It's the nearest thing we can get in this sort of situation to gaining confidence by verifying a theory's prediction experimentally.