Armando Iannucci: It's time for Chilcot's team to flex their ageing muscles

"It's gripping television." "Nothing much happens, but that's the beauty of it." "Sometimes the most dramatic thing is not what the characters say, but what they leave unsaid." It could be a description of Mad Men, the cult US series about advertisers in the 1960s, but it's actually the evolving consensus on the Chilcot inquiry, the cult probe being watched on laptops and TV sets across the land. What's drawing the crowds is a dawning sense that this process might just get to the bottom of what actually happened before British troops invaded Iraq seven years ago.

Despite the disastrous failings of intelligence, the obvious lack of preparedeness and the horrendous whiff of deceit, no one so far has apologised or got sacked. Heads did not roll; they got knighted. Now, it seems, Chilcot is coming closer to fingering culprits, in full view of the media and the curious and occasionally aggrieved public.

It's a surprise. The panel consists of firmly entrenched members of the Establishment. We were warned that Chilcot himself was going to be another yes-man appointed to guarantee that an independent inquiry would give the Government the independent conclusion it wanted. Who, after all, can forget Lord Hutton – a man genetically predisposed towards assuming that rocking the boat and disrupting the process of government was tantamount to treachery? The reason Hutton is now historically laughable is that his conclusion so mismatched the weight of factual evidence set in front of him that it seemed like the product of an entirely different process.

The question then is: is Chilcot Hutton reincarnated? First signs weren't encouraging. There are no lawyers on the panel. Witnesses have said potentially explosive things but, instead of anyone on the inquiry team spurting out their water and shouting "That sounds utterly indefensible," they've often as not responded with "Thank you very much and mind how you go: the roads are icy." A key moment for me was when Lord Goldsmith appeared to suggest that his complete, convenient change of heart on the legality of the war came over a chance lunch with a French diplomat. Hardly the thorough fact-finding on which the great events of history are determined.

It's at points like this that you are reminded of the strange disconnection between the official version of events and the reality we all fear and some of us know. That suspicion that while Blair was telling us no decision had been made, he was behaving like someone who'd made his mind up months before. That uncomfortable behaviour from ministers when they said they were solidly behind the PM while looking as if they were anything but. Those vocalised expressions of certainty that drowned out quiet doubts that all wasn't quite what it seemed.

Talk to anyone who worked in Whitehall at the time and you hear far, far worse than anything that's emerged in an official inquiry. Researching my film In The Loop, a comedy about a British prime minister and a US president in the lead-up to an invasion of a Middle Eastern country, I talked to civil servants, advisers and diplomats in London and Washington. They told me tummy-churning stuff. Of Blair being so excited at being in the Oval Office he nearly hyperventilated. I heard of how doubts about the legality of invasion were the closest the British military had come to mutiny. Of how the Pentagon tried to freeze out the State Department by speaking at joint meetings entirely in acronyms that only Pentagon staff would understand. And of how Donald Rumsfeld weeded out from those going to help the reconstruction of Iraq anyone who could speak Arabic, on the grounds they would be pro-Arab. As a result, it took the Americans 18 months to realise that when marines held up the flat of their hand to oncoming cars to signal them to stop, they were actually using the Iraqi hand-signal for "come forward". That's why so many families in cars were shot.

The frustration is that once people are up in front of inquiries, they clam up. Absolute private fury becomes "I was not best pleased" in public. Utter incompetence translates into "most surprising" behaviour. The challenge for Chilcot is whether he has the will to decode and transmit this mandarin-speak into plain English. So far, slowly, painfully slowly, Chilcot and his team have been flexing their aged muscles. Attempts to classify vital documents have been ridiculed. A more sinister attempt to keep the proceedings behind closed doors was repulsed. And most key witnesses have been given time to lay their own traps. My favourite is Alastair Campbell's "I'm clarifying the answer I thought I gave to the question I thought I was being asked."

We now know Goldsmith changes his views on legality, even though Blair has publicly said this account is nonsense. We now know intelligence chiefs said intelligence was patchy, while Blair said it was "beyond doubt". And we've been hearing claims that Blair's mind was made up for invasion maybe a year before the "final" decision was given to Parliament.

Today is the day the final connections can be made. Time and time again in this inquiry, ultra-tough questioning has been stalled with a simple "Well, you'll have to ask Mr Blair that." This phrase is fast becoming the inquiry's catchphrase. The piles of difficult questions Chilcot will have to ask Mr Blair have been mounting. Today is the only chance his team gets to ask them. By the end of the day, we'll know whether Chilcot is Hutton regenerated, or something far more potent. It's the day the Establishment finds out whether he is a mandarin or a monster.

Armando Iannucci is a writer and film-maker who produced the television series The Day Today and created The Thick of It