I occasionally like to kid myself that after 20-plus years of reporting from Westminster, I can’t be entirely surprised any more.

Then, along came the row of the last few days after The Guardian’s revelations about Lord Mandelson, his vetting and the removal of the most senior civil servant at the Foreign Office, Sir Olly Robbins.

It all prompted a one-word question from many of us: how?

How could so senior a civil servant be in possession of such apparently politically explosive information and not pass it on to the prime minister and others?

And how could a prime minister, embarking on a high-profile and controversial appointment, be so insufficiently curious that he didn’t prise this information out of the government machine?

Context is key here: in January of last year, Sir Olly had just arrived at the Foreign Office as its permanent under-secretary.

Weeks before, his predecessor, Sir Philip Barton, had worked through the formal niceties of confirming Lord Mandelson’s appointment to Washington, such as writing to the King about it on the 18th of December.

In other words, it was a done deal. We already know some vetting issues had already been raised with the prime minister, but Whitehall, the civil service, knew of Sir Keir Starmer’s wishes.

So perhaps there was a sense that the security vetting didn’t reveal much more than was already known, convention said it shouldn’t be passed on anyway, plus it was too late.

This would appear to be the bones of the argument Sir Olly will use publicly when he appears before the Foreign Affairs Select Committee.

And this is where we get to the pointy headed end of this row: a debate about the meaning of the Constitutional Reform and Governance Act 2010.

It states that “the secretary of state has the power to manage the diplomatic service…but they do not cover national security vetting.”

This is a reference to the foreign secretary and outwardly gives cover to Sir Olly’s anticipated defence, that the law states he should be discreet with information gathered during vetting.

And yet on Sunday night, the government published a statement setting out its interpretation of the law, claiming nothing stops civil servants from “sensibly flagging UK Security Vetting recommendations.” They even delve into the explanatory notes of the Act to try to make their case.

I suspect the prime minister may also make reference to the Civil Service Code.

This spells out that civil servants must not “deceive or knowingly mislead ministers, Parliament or others” and “civil servants advising ministers should be aware of the constitutional significance of Parliament, and of the conventions governing the relationship between Parliament and the government.”

In other words, don’t let ministers go before MPs and say things you know to be a misleading picture.

In short, Sir Keir will say I wasn’t told and the fact that Sir Olly’s friends are saying he was right not to tell him proves the prime minister didn’t lie.

The Conservative argument in the Commons will amount to suggesting that Sir Keir has beheaded more people close to him than Henry VIII – and they hope Monday afternoon will “hasten squeaky bum time of Labour MPs”, as one Tory source put it to me.

Kemi Badenoch will challenge Labour MPs to ask themselves if Sir Keir is the best they can do, as the government finds itself consumed again by questions about Lord Mandelson as opposed to the consequences of the Iran war, the cost of living and much else besides.

The mood within the Labour Party had changed in recent weeks, it has felt less volcanically angry. The question now is whether this fiasco turns the temperature up again.

“Let’s be frank, it’s bad this stuff,” one said to me. “It reignites everything so many were furious with him about in the first place,” says another. With a big set of elections just over a fortnight away, timing couldn’t be worse for them.

Meanwhile, the Financial Times is reporting that Sir Olly is contemplating legal action against his dismissal.

Perhaps little wonder given he was so spectacularly fired out of a cannon by the prime minister and foreign secretary, over the appointment of an ambassador that had been signed off before Sir Olly even arrived at the Foreign Office.

Those who have worked alongside him describe him as a “by the book man” who has had a distinguished career in public service and is “diligent about processes and acting properly.” The tone and tenor of his appearance before the Foreign Affairs Committee will be as fascinating as his testimony.

It will also be worth cross-checking what we hear from the prime minister on Monday afternoon with what has already been published about the appointment of Lord Mandelson.

Several people I spoke to over the weekend said the ambassador in Washington would typically have the highest level of security clearance – what is known as Strap 3.

The Times is reporting that Lord Mandelson did indeed have this level of clearance and it is my understanding that while he was in Washington he was frequently taken to a secure room in the embassy to read this material, as is standard practice with the most sensitive intelligence.

And yet, in the deluge of documents already published there is an email sent to Lord Mandelson at the beginning of February of last year, when he was beginning in the role, saying, on page 76, that the job “requires DV+STRAP” and so “a new STRAP application form will need to be submitted as STRAP clearance is role specific”.

A further document sent to him on the same day says “you should submit a new STRAP application form for your new position, at least 3 months before you are due to start at Post”.

This was sent to him as he was beginning the posting, not months earlier and it doesn’t appear there were any restrictions placed on what Lord Mandelson could see in his early months in the job.

There are so many questions – and an awkward few hours ahead for the prime minister.