The Brexit negotiations are like one of those ornate mazes that insist on returning you to the same place.  This morning, they are revisiting one of their familiar dead ends: the Northern Ireland-only backstop.

Its origins lie in the necessity of a border if Brexit is to take place.  Unionists and the Government want part of it to be the land border between the UK and the Irish Republic.  Irish Nationalists, the EU and the Irish Government want that part in the Irish Sea instead, in effect keeping Northern Ireland in the Single Market and Customs Union.  Both sides praise the Belfast Agreement and condemn a hard border.

Theresa May originally opposed either a regulatory or customs border in the sea.  The Withdrawal Agreement saw her U-turn on the first but stay her course on the second – up to a point.  It accepted that Northern Ireland remain in the Single Market, but placed the entire UK in a customs union.  The Brexit settlement would thus see new regulatory barriers in the Irish Sea, but not a customs one.

Or at least not one at first.  For while Great Britain would be in a customs union, Northern Ireland would be in the Customs Union.  It followed that the first might fall away at some point in the future – leaving the two parts of the UK in different customs jurisdictions after all.  These arrangements added up to the backstop proper.  Unionist resistance to the proposal played a big part in defeating the Withdrawal Agreement three times.

And so the negotiation comes back once again to the original EU proposal – a Northern Ireland-only backstop.  It has four main opponents at Westminster.

First, the DUP, who are unhappy already with the plan to keep Northern Ireland in the Single Market while the rest of the UK is kept out of it, and don’t want to see the same arrangement apply to the Customs Union.

Second, pro-Union Conservative MPs, whose take is the same.  (It’s important not to assume that all pro-Brexit Tory MPs are also pro-Union: some have English nationalist leanings.)

Third, Scottish Tory MPs, who are concerned that Nicola Sturgeon will seize on Northern Ireland being separated from the rest of the UK in this way, and will demand the same treatment for Scotland.

Finally, the Labour Party, which wants the UK to stay in the Customs Union.  It has been opposed to Great Britain being out of it, and so all set up for a low-alignment, “Canada-dry”-type future relationship with the EU.

Now it will be claimed at this point that Boris Johnson is opposed to a Northern Ireland-only backstop, since he now says that he is, but he is clearly pushing for a kind of distant cousin of the scheme.  As a starting-point, he is proposing that animal and plant checks be put into an all-Ireland zone.  It is possible to glimpse, through the swirling mists of speculation, what agreement might look like.

In essence, there would be some further all-Ireland alignment; some checks away from the land border itself under “alternative arrangements”; some oversight at Stormont for some of this settlements, and perhaps a blurring of the lines, by some means, over customs and regulatory jurisdictions.  The last will be a formidable challenge to the inventiveness of the negotiators, assuming that both the EK and EU are really up for a quick deal now.

After all, the backstop problem seems to have no solution – since most Unionists don’t want Northern Ireland to be in the Single Market and Customs Union, but the Irish Republic can’t be taken out of either and stay an EU member.  But the same was said of the Troubles, and the Belfast Agreement somehow held some red-white-and-blue content (the consent principle, say) within a green wrapping (the concept of “the people of the island of Ireland”).

All this is a rough sketch of what aficionados of the negotiations call “the landing zone”.  But there is a danger that such a plane would crash – if it can fly in the first place – and Johnson not emerge from the wreckage.

For the logic of such a solution would turn on its head much of what has happened since Parliament returned and was prorogued.

First of all, the DUP would presumably be on board, having obtained the guarantees that they want about Stormont oversight of the settlement.

Next would come most of the 21 former Tory MPs from whom the whip has been removed. They would back the plan and have whip restored.  The Conservative Party as we have known it would be back in business.

Then, just possibly, some Labour MPs would join independents to vote for the proposal: the Lisa Nandys and Stephen Kinnocks and Caroline Flints and 17 Labour members of the new MPs for a Deal group.

That would leave not Remainers (or Soft Brexiteers) in defiance of the Tory whips, but Leavers – the so-called “Spartans”. So out the latter would go.  Welcome back, Philip Hammond.  Goodbye, John Redwood (say).

And if the Spartans were still whipless when the general election came, it would be their seats that would see new Conservative candidates, not those of the Remainers.  In very crude terms, the purge would apply to the right of the party, not the left – a prospect we sketched out when this month opened.  Johnson would be campaigning as a One Nation Tory in the current distorted use of the term (it is widely used simply as a synonym for pro-EU politics).

In the aftermath of a Scottish court ruling that the Prime Minister’s prorogation is unlawful; as Opposition MPs clamour to see the text messages of Special Advisers; while the Government is forced to publish Operation Yellowhammer details, and as some of those 21 former Conservative MPs snipe at Downing Street, such an about-turn sounds fantastically unlikely.  And it remains the case that a deal is very unlikely indeed.

But always be ready for the unexpected.  In the event of a deal based on a Northern Ireland agreement of this kind, Number Ten’s snub to Nigel Farage yesterday would make sense.  Johnson would claim to have delivered Brexit.  Farage would say that he had not.  The Prime Minister would be fighting an electoral war on two fronts.  If this site’s surveys are anything to go by, many party activists would be very unhappy indeed.