Here is the conclusion of a three page legal opinion from Cox.

“15. In my letter of 13 November 2018, I advised that the Protocol would endure indefinitely in international law and could not be brought to an end in the absence of a subsequent agreement. This would remain the case even if parties were still negotiating many years later, and even if the parties believed that talks have clearly broken down and there was no prospect of a future relationship agreement.

16. I also advised that in the specific case that situation was due to the EU’s want of good faith and best endeavours, because of the difficulties of proof and the egregious nature of the conduct that would be required to establish a breach by the EU of those obligations, it would be highly unlikely that the United Kingdom could take advantage of the remedies available to it for such a breach under the Withdrawal Agreement.

17.I now consider that the legally binding provisions of the Joint Instrument and the content of the Unilateral Declaration reduce the risk that the United Kingdom could be indefinitely and involuntarily detained within the Protocol’s provisions at least in so far as that situation had been brought about by the bad faith or want of best endeavours of the EU.

18.It may be thought that if both parties deploy a sincere desire to reach agreement and the necessary diligence, flexibility and goodwill implied by the amplified duties set out in the Joint Instrument, it is highly unlikely that a satisfactory subsequent agreement to replace the Protocol will not be concluded. But as I have previously advised, that is a political judgment, which, given the mutual incentives of the parties and the available options and competing risks, I remain strongly of the view it is right to make.

19. However, the legal risk remains unchanged that if through no such demonstrable failure of either party, but simply because of intractable differences, that situation does arise, the United Kingdom would have, at least while the fundamental circumstances remained the same, no internationally lawful means of exiting the Protocol’s arrangements, save by agreement.”

Theresa May will stress part of paragraph 17 – that the revised deal “reduces the risk” of the UK being trapped in the backstop against its will.

But that must be read in the context of the end of paragraph 19 – that ” the United Kingdom would have, at least while the fundamental circumstances remained the same, no internationally lawful means of exiting the Protocol’s arrangements, save by agreement.”  In other words, no unilateral right to leave.

The Prime Minister will look for comfort in the suggestion that fundamental circustances might not remain the same.  But that is a very thin picking.

You may ask how on earth Cox can first do his own homework, then mark it – and find it wanting.  The answer is clear enough.  He wasn’t in Brussels when the revised deal was finally nailed down and signed off.  The UK and EU civil service negotiators were in charge.  It isn’t his homework at all.

As we write, it looks as though Cox has done his duty both as a politician (he will of course vote for the deal) and as a lawyer (which in our vulgar reading means that one can bend the meaning of words, but not break them).

Cynics will write that he had no choice – that no other course would preserve his reputation at the bar; that he would not have wanted, as we say in the trade, “to do a Goldsmith”.

This site takes a less jaundiced view.  Cox set up his own legal firm – Thomas More Chambers.  He will have read, and seems to have learned from, Robert Bolt’s Man for All Seasons, in which More says –

“When a man takes an oath…he is holding his own self in his own hands. Like Water. And, if he opens his fingers then – he needn’t hope to find himself again.”