Now that the Prime Minister won the breathing space she needs to bring forward ‘MVIII’ – the next in the Government’s ongoing series of decisive votes on Brexit – she must return to the thorny issue of how to win it.

As her strategy is currently to maintain the unity of the Conservative Party and the broad thrust of its 2017 manifesto, she needs to do this by winning over the substantial bloc of Tory MPs holding out against the deal.

The good news is that a number of those who rebelled on the first Meaningful Vote came home on the second, and there are many signs that more are keen for an opportunity to do so. Indeed, the signal failure of the bid to ‘amend’ the deal was that Geoffrey Cox didn’t feel able to provide these MPs with adequate reassurances even though many of them really wanted him to.

In a debate on the legal advice Jacob Rees-Mogg invited Steve Barclay, the Brexit Secretary, to offer him reassurance on this point. (This brief exchange is worth reading, for we’ll return to it).

Meanwhile the Times reports one “senior Brexiteer” said that many of the European Research Group could have voted with the Government on the second meaningful vote had Cox referred to the Vienna Convention in his advice, charging him with “incompetence”.

But you can forgive those who are now sceptical about the Attorney General’s new proposal to use the Vienna Convention on the Law of Treaties, to give it its full title, as Britain’s backstop get-out clause, and not just because Cox didn’t feel able to place great weight on it before political necessity forced his hand.

The Convention itself, as the BBC explains, essentially states that parties to a treaty have the right to withdraw from it in the event of relevant circumstances changing significantly in a manner the signatories could not foresee.

However, Martin Howe, one of the Brexiteers’ legal experts and a member of the European Research Group’s so-called ‘Star Chamber’ which rejected Cox’s advice, thinks this is a complete non-starter, telling the Evening Standard that:

“The leading case in the International Court of Justice (the Gabcikovo-Nagymaros case between Hungary and Slovakia) shows this requires radical change of circumstances. The fall of the Soviet Union, disappearance of the Warsaw Pact, and dissolution of Czechoslovakia, were not sufficient to satisfy this ground.

“The other issue is, under Article 62, the change of circumstances has to be unforeseeable. As we are talking about this ‘change of circumstances’ now, it cannot be unforeseeable.”

However Lord Pannick QC, a constitutional lawyer (and supporter of a second referendum) takes a different view, writing in a letter to the Times:

“Article 1 (4) of the Protocol on Ireland/Northern Ireland states that the objective of the withdrawal agreement “is not to establish a permanent relationship between the Union and the UK”.

“If, therefore, the UK and the EU were unable to reach an agreement on Northern Ireland/Ireland, despite good faith negotiations and despite the arbitration procedures, and if the UK were therefore to be faced (against its will) with a permanent backstop arrangement, the UK would be entitled to terminate the withdrawal agreement under Article 62 of the Vienna convention on the Law of Treaties.”

Another complication is that, as this note from the House of Commons library points out, the Vienna Convention only covers treaties between states, whereas the EU is an international organisation and “the Vienna Convention involving international organisations is not yet in force.”

But advocates of this approach – notably Sir Edward Leigh, who tabled an amendment on it – seem to believe that the Convention will nonetheless provide what the HoCL calls “a very strong guide” on the relevant international law. That may be so, but it makes an already very slender ladder from Cox slenderer still – and that’s before taking into account the political likelihood of taking such a decisive approach when the time comes.

In light of all the above, here is the circumstances in which Barclay set out that the United Kingdom could, hypothetically, use Article 62 of the Vienna Convention to depart the backstop:

“…if the United Kingdom took the reasonable view, on clear evidence, that the objectives of the protocol were no longer being proportionately served by its provisions because, for example, it was no longer protecting the 1998 agreement in all its dimensions”.

On the surface, this ties up with Cox’s other thinking about the Belfast Agreement, which we have covered elsewhere. But apply Martin Howe’s reasoning to the above statement and that sense evaporates.

First, Barclay’s circumstances appear require the treaty to become a threat to the Belfast Agreement – being one now, which Cox appears to think it might be, is not a change in circumstances. Think about the dominant narrative surrounding the Agreement and mull how likely that is.

Second, in order to qualify under the convention such a threat needs to be unforeseen, meaning almost by definition that concerned Brexiteers can’t think up such circumstances now. Indeed, any such effort would be self-defeating.

The technical possibility of the stars aligning in a totally unforeseen and hugely improbable way is not a unilateral exit from the backstop so much as the outside chance of an act of God. Little wonder Rees-Mogg wasn’t convinced.

In the end, though, it may be enough. If enough Brexiteer MPs share Simon Clarke’s view that they are “increasingly operating with a gun to our head”, ‘Cox’s Viennese Codpiece’ may yet provide just about enough cover for a retreat.