Theresa May's original hope and plan to move Brexit talks on to phase two was simply to say to the rest of the EU that she crossed her heart and hoped to die that the border between Northern Ireland and the Republic would be kept open and wholly permeable forever.

And to give further reassurance, she would have pledged that in the event that Brexit turns out to be of the abrupt and hard variety, her absolute top priority would be to make sure a hard border would not be reinstated. Such a hard border was (is) the reddest of her red lines.

But Dublin and Brussels would not take her word as her bond. So she had to come up with a practical plan to keep the border invisible (to all intents and purposes).

The Irish border issue has become a sticking point in Brexit talks. Credit: PA

So the document that was agreed by the Irish PM Leo Varadkar and the EU Commission president Jean-Claude Juncker had three border scenarios.

Scenario "A" said the border would be kept open via a trade deal, to be negotiated in those all-important phase two talks.

However if those trade talks never happened or did not involve frictionless trade between the UK and the rest of the EU, or trade without customs checks, there could possibly be a technological solution - known as Scenario "B" - whereby higher value goods traffic would be screened by the magic of digital technology, with only the rarest of human and traffic-delaying intervention.

Theresa May needs to put a plan to her Cabinet. Credit: PA

Finally if the rest of the EU was not seduced by that tech plan, there was Scenario "C", the one that said in a worst case (for the UK), there would be regulatory "alignment" between the Republic and Northern Ireland, thus obviating the need for most customs checks (as per my last three or four posts).

Now to the frustration of the PM, all the fuss and bother has been about Scenario C, the one she neither wants nor thinks is likely. Gosh, international politics is so annoying.

The thing is, like it or not, she has to sort Scenario C, and before Sunday night - which is the cut-off time for agreeing a comprehensive phase-one-Brexit deal that could then be distributed by Donald Tusk for scrutiny by all 27 EU member states in time for their approval (or rejection) at the end of next week.

Leo Vardakar, left, has three scenarios. Credit: PA

Can such a Scenario C breakthrough be found? As I've been saying for a couple of days - and possibly it's because I keep dodgy company - the only seemingly workable solution that has been put to me is the one where regulatory alignment applies to the whole of the UK and not just Northern Ireland.

Is that the spell that will move Brexit talks on from Christmas past and present to Christmas future, a shiny transition deal and post-Brexit trade and security arrangements? It might.

But she'd have to get it through her Cabinet. And what is perhaps a sign she's not confident of her ministers' backing, she did not put that plan to them on Tuesday morning, Yikes!