If you wanted to know what really went on in the run-up to the Iraq war, Matthew Rycroft would be the person to ask. He was Tony Blair’s private secretary for foreign affairs from 2002-4 and saw just about everything that happened at first hand. No doubt that is why the Iraq inquiry has just seen him in secret.

The inquiry has just published an update on what it has been doing since public hearings ended in July. It visited Iraq, as promised, and has published the names of some of the people it spoke to, but not what they said. It has also revealed that it saw two witnesses in what it insists on calling “private hearings”. Of the two, Rycroft is undoubtedly the more significant.

What is intriguing about Rycroft’s secret session is that we are not told why. The inquiry coyly points to its protocol on witness evidence, which states that most witnesses will be seen in public but sets out reasons for secret hearings. These include the usual issues of “national security” and “vital national interests” but also “to protect any [junior official] who may wish to give evidence that runs counter to others”.

This is the Iraq inquiry in a nutshell. Are they trying to sit on sensitive information to protect the British state from embarrassment? Or trying to make it easier for people to blow the whistle? We won’t know until the report is published — early next year — and even then we won’t know what, if anything, we are not being told.

What we do know is that Rycroft would be the perfect whistleblower. He wrote many of the documents that the inquiry has failed to publish and was at Blair’s side at virtually all of the key meetings. He was, for example, the author of the notorious Downing Street memo, which recorded a crucial meeting at No 10 in July 2002, and was present at the White House six months later when Blair told George Bush, that Britain was solidly behind the war, whatever the outcome of UN inspections. He saw the very unwelcome advice from attorney general Lord Goldsmith a day earlier — that war would be illegal without a new UN resolution — and apparently wrote on the memo: “specifically said we did not need further advice [on] this matter.”

But if Rycroft has spilled the beans, it is far from clear whether his evidence will see the light of day. The inquiry is still dithering about whether to call back key witnesses, like Blair, to go through any gaps and contradictions — which could now include what Rycroft has said. And in its press release today, it claims that its protocol “sets out the approach the Inquiry will take to considering how best to draw on and explain in public what was covered in private”. Except that all the protocol says is that the inquiry will “careful consideration” as to how best to do this.

If Rycroft’s evidence does not feature, we’ll have to draw our own conclusions.