On the day Boris Johnson became prime minister, I said his most important appointment was that of Dominic Cummings - who had run the triumphant pro-Brexit, Vote Leave referendum campaign - as his most senior government adviser.

It signalled Johnson was not bluffing when he pledged to extract the UK from the EU do or die, no ifs no buts, deal or no deal - because there are few political operators on the planet more ruthless, focused and remorseless than Cummings. I imagine he has “OODA” tattooed on his bottom (look it up).

There has however been a bit of a misunderstanding about precisely what Cummings agreed to do for Johnson and for how long. So here are the terms of their compact.

When on the evening of Sunday 21 July, Johnson formally offered Cummings the position of senior consigliere - an offer that was not quite out of the blue, but which Cummings had not really been expecting - Cummings took on the job as largely a single short-term project, to deliver Brexit by the due date.

Dominic Cummings pictured on the right of picture as Boris entered Downing Street as prime minister. Credit: PA

Or to put it more prosaically, his agreement to go into 10 Downing Street expires at 11pm on 31 October, do or die, as it were.

Cummings had to make a proper sacrifice to take the job; Johnson talked him into cancelling a surgical procedure, serious enough to warrant general anesthetic, which had been scheduled for three days later, when Johnson formally became PM.

He promised his wife, the journalist Mary Wakefield, that he would reschedule the operation for the week following 31 October and only after the op would he then discuss with Johnson what - if any - his future role in government would be.

I cannot judge whether Cummings will return after the surgery. I understand it depends on his health, the views of his wife, and whether he and Johnson can agree on a long-term role that would suit them both. Pimpernel-like, he may vanish again.

Until that moment of truth, he is concentrating on making a reality of Brexit - which for him is a point of honour, since he played such a significant role in winning the vote to leave the EU (which is why his spouse instructed him to take Johnson’s offer).

Prime Minister Boris Johnson. Credit: PA

He is also doing what he can to make a reality of his other obsession, which is to foment a cultural revolution in Whitehall so that civil servants have a more instinctive grasp of the importance of science and technology to the UK’s future.

For Whitehall and his political advisers, his advent is something of a mixed blessing - even if like him they feel a sense of duty to make a reality of the referendum result (part of the challenge he faces is that not all do). He has “cancelled everyone's holidays” - in the words of a source - and he is working every day from breakfast till midnight until October 31st

What should be crystal clear - from his Stakhanovite dedication, the daily Brexit planning meetings in the COBR emergency room, the twice weekly meetings of the Brexit kitchen cabinet - is that he is in deadly earnest about taking the UK out of the EU without a deal, that it is not a “negotiating bluff” as some believe.

That said - and this is clear from everything he said during the referendum campaign and since - he would prefer a negotiated Brexit. And as I understand it, for Cummings and Johnson the only condition of a deal they won’t compromise is that the EU must abandon the Northern Ireland backstop (I say “only” - but for the EU and Dublin this is the reddest of red lines).

Dominic Cummings gives evidence to the Treasury Select Committee in 2016. Credit: PA

There is no chance of any negotiating breakthrough this side of 20 September, or indeed of any serious negotiations taking place between Johnson and the EU till the end of September- because the likes of Tony Blair and the rebel Tory MP Dominic Grieve have persuaded EU leaders that when MPs return to the Commons on 3 September they will find a way to block a no deal Brexit.

I have spoken and written extensively about how MPs have the power to frustrate no deal (through legislation annulling a no-deal Brexit as the default option, or bringing down the Johnson administration) but in my view will fail to do so. I will return to all that in a further note soon.

Suffice to say that my central planning assumption is that there will be a general election in the autumn, and that the decisive battle between executive and legislature, between Johnson and opposition MPs, will be over whether polling day is before or after Brexit day (so early November, if Johnson gets his way, or late October).

If when party conference season is in full swing, Johnson still seems on course to execute a no-deal Brexit, there is a hope in the government - no more than a hope however - that Paris, Berlin, Brussels and Dublin would at that critical juncture agree to dump the backstop and countenance some “light” friction in the passage of goods and foods back and forth between Northern Ireland and the Republic of Ireland.

Failing that, there is a dream the EU could at the end of October belatedly agree those tantalising “mini deals” to soften the shock of a no-deal Brexit.

All of which may be - almost certainly is - wishful thinking.

But equally it is magical thinking to assume Cummings will lose his nerve and slam the brakes on the no-deal juggernaut.

He won’t.

So in my estimation, for those who are terrified of a no-deal Brexit, the question is not whether Cummings will blink. He is psychologically and physiologically unequipped to do so.

It is whether Johnson will hold his nerve and keep himself handcuffed to Cummings’s steering wheel, till this fateful journey is done.