The suggestion is that Number 11 and Number 10 did not check the Budget against the Tory manifesto of 2015, according to Paul Goodman, the editor of Conservative Home. They seem to have walked into breaking the manifesto commitment not to raise National Insurance Contributions. How can that be?

It makes no sense. Was Hammond too focussed on perfecting his jokes? Two Tory MPs I called who might know were baffled, with one saying they thought it was mad but would have to wait and see whether Number 10 had an answer.

The positive spin is that this is a move that needs to be made, with self-employment having increased to such an extent that it will only punch a bigger hole in government revenues as the years go by.

But one could say the same of the daft “triple lock” which gives affluent pensioners winter fuel allowance to spend on warming themselves by drinking cases of wine. Poorer pensioners obviously spend it on heating. The government is stuck with that promise because of the commitments the Tories made in the 2015 general election under David Cameron, now departed. Yet it is somehow fine to abandon pledges made to millions of self-employed people?

The Number 10 position, if I understand it correctly, is that (eyes glaze over) the manifesto promise has been delivered somehow and (wake up!) they were talking about one class of NIC and not another. Yeah, that’ll work! Across the land, in pubs tonight, self-employed people will shrug and say: “Oh, that’s ok. They meant class one of NIC, not class four, or is it class one? No matter! Much obliged. Take one for yourself, Steve Barman, and send the change to Philip Hammond.”

What I’m struggling, genuinely, to get my head round is how the conversation went when the budget was being discussed in Downing Street before its delivery.

Theresa May: “Ok, and what’s this page on national insurance and self-employment?”

Philip Hammond: “Have you heard the one about George Osborne, David Cameron, Boris Johnson and Michael Gove stuck in a lift at Davos. Well…”

Theresa May: “Back to the budget. Get on with it.”

Hammond: “Right. The self-employed.”

May: “We’re in favour of them? Enterprise Britain and all that? Aren’t we?”

Hammond: “Up to a point, Prime Minister. They’ve had it too easy for too long. We need a more level playing field and…”

May: “You want to put up their national insurance?”

Hammond: “Yes.”

May: “What does the manifesto you and I got elected on say about National Insurance?”

Hammond: “I…”

May: “Philip. What on earth are you thinking?”

Hammond: “Why did the chicken cross the road?”

If it did not run something like that, without the Hammond jokes, then why not and what were those involved playing at? There has been much talk recently of a dysfunctional relationship between Number 10 and Number 11 and of tensions behind the scenes, with Cabinet Secretary Jeremy Heywood trying to broker compromises. Perhaps those tensions diverted energy from the basic work? Did May miss the NIC change? Was Hammond, convinced he is unsackable, a shade too relaxed? Perhaps they both got carried away with the collapse of Labour and didn’t twig.

It is bizarre. Answers on social media, please. Or by text if you’re a Tory MP.