Peter Kosminsky remembers the day he was asked to make a film about David Kelly. It was July 24th, 2003, the day of his mum and dad's 50th wedding anniversary. It ought to have been a very happy day. But his father had died a few weeks before, leaving  nothing to celebrate.

The image of him on his deathbed, mouth partly open, half-lidded eyes staring up at me, haunted my sleeping and waking dreams. I suppose I was starting to realise that I would miss him very much.

I had gone to stay in the Languedoc at the house of a friend to regroup. It stands on a bluff in what is known as the Valley of Winds. Above it lies a ruined Cathar castle. Below it are a dozen ancient stone cottages - their arched, terracotta roof-tiles flowing down the hillside in waves. Even in July, Pyrenean winds lay siege to the house, rattling the bottle-green shutters by day and night.

I remember the moment quite vividly. I was standing on the balcony. In a gap between the houses below, an elderly man stared up at me. The hairs stood up on the back of my neck. He bore an astonishing resemblance to my father, as he had looked before his illness. I watched him, afraid to blink or look away in case he vanished. But he remained firmly corporeal, staring up at me with a neutral expression. That's when the phone rang with the offer of the job.

Now, almost two years later, I can't be sure whether my decision to accept was influenced by memories of my dead father. It seems likely that it was. In purely practical terms, however, I was the worst possible person Channel 4 and the production company Mentorn could have chosen to make a film about David Kelly and Why We Went to War. Researching it would require the active assistance of the government, and my relations with No 10 were atrocious. The Project, my last film for television, had been a BBC drama about the Labour party that was most notable for the fact that Labour refused to have anything to do with it.

The writer of that film, the late and much-missed Leigh Jackson, had wanted to write about how Labour had won the 1997 election and what had happened to its army of young activists once power had been achieved. Our first step had been to approach as many of those activists as possible. There was only one problem - none of them would talk to us. We tried ministers, advisers to ministers, MPs, members of the Policy Unit, the Rapid Rebuttal Unit, the Media Monitoring Unit, the Key Seats Unit, the Last Seven Days Unit, pretty much every kind of unit that had existed in Millbank in the run-up to the 1997 election. In every case, the answer was no.

In the end, one Millbanker took pity on us and let us into the secret. A letter had gone out from Alastair Campbell's department to all who worked for the party - don't talk to Kosminsky.

Eventually, I was permitted a cup of tea with Lance Price, the author of the letter and then Labour's director of communications. I suggested to him that the party's policy was self-defeating. Hard as he might find it to believe, we were approaching the subject with an open mind, but his strategy of silence was driving us into the clammy embrace of the disaffected, those who were more than happy to speak because they'd been sacked or left Labour under a cloud. I needed Price's help to achieve some balance.

He looked pained, and sipped his lukewarm tea. Later, he left his job and retreated to a farmhouse in France.

The whole sorry episode ended when, after transmission of The Project, I appeared on Newsnight with David Hill. Hill was then a political consultant but, in the run-up to the 1997 election, had been Labour's chief press officer at Millbank. We had written to him on two occasions asking him to talk to us, without success. Now I had to sit and listen while he told Kirsty Wark how wrong we had got it. And how he had never been approached to advise on what the programme might contain.

Like most of the nation, my first contact with David Kelly was through the medium of television. I sat at home and watched him give evidence to the foreign affairs select committee on an airless summer's day in July 2003. There had been a bomb scare in Westminster and Kelly had had to make his way to the Commons on foot, running the media gauntlet at St Stephen's Gate. As a result, he kept the "high court of parliament" (as Andrew MacKinlay was later so famously to put it) waiting, and was flustered and on edge when he finally sat down at 3.05pm. But neither the dash from the cabinet office nor the fact that the fans in the committee room seemed to drown out many of his words could explain the discomfort we witnessed that day.

It was as if a creature used to the shadows had been revealed, squinting, by the over-turning of a stone. And yet Kelly was used to the media spotlight - he had given interviews while a weapons inspector in Iraq in the 1990s and knew what it was to be questioned on camera. Why did he seem so ill-at-ease, so uncomfortable with the role he had to play? Why did some of his answers seem to support the government in its war with the BBC, while others were actively unhelpful? I suppose those questions were still somewhere in my mind when, three days later, I watched Tony Blair's chastened reaction to the news that his government's chief weapons inspector had been found dead in the woods.

The prime minister asked Lord Hutton, a former judge in the non-jury Diplock courts of Northern Ireland, to investigate the circumstances surrounding Kelly's death. Clearly our first priority would be to absorb the vast amount of material certain to be thrown up by his inquiry. For that reason, we attended every day of Hutton and read every one of the 10,000 pages he posted on the Internet.

But Hutton, perhaps inevitably, skated over many of the themes that would preoccupy a drama - character, motivation, the minutiae of daily life. And he was almost completely silent on Kelly's seven years as a weapons inspector in Iraq, years that seemed to me, increasingly, to be key to understanding why Kelly sat down with Andrew Gilligan at the Charing Cross Hotel in May 2003. So we would have to conduct our own interviews, and for that we would need the help and support of the government.

Initially, the signs were encouraging. The Hutton report might have criticised the BBC, but it had exonerated the government utterly. The prime minister's declared approach to the death of Kelly had been one of openness and transparency. Since we seemed likely to be the one drama made for British television about that subject, there was every reason to hope that Downing Street would assist us in constructing as accurate a picture as possible. And there ought to have been one, even mightier factor in our favour. The same David Hill who had regretted the fact that we hadn't come to him for help on The Project was now director of communications at No 10. But this was the response.

"Dear Mr Chinn [Simon Chinn, our co-producer], Thank you for your letter . . . We have no inclination through the medium of the drama you are producing, to further engage on the issues you intend to cover. Yours sincerely, David Hill."

I wrote to Hill, reminding him of our conversation on Newsnight and asking him to reconsider. I pointed out that it was hardly fair to criticise the media for getting it wrong on transmission when requests for help and guidance at an early stage are greeted with a blank rejection. I awaited a reply. None came.

Next, Chinn tried to contact Campbell to see if support from him would unlock the door. This is a rough note of their brief conversation:

SC: "Hi, I'm calling from Mentorn, which I expect you know of from Question Time. I sent you a letter on June 7th about a drama we're producing on the David Kelly affair. I expect you receive lots of mail but . . ."

AC: "Oh yes, I remember. The answer is no."

SC: "Um, but I didn't askyou a question."

AC: "Yes, but I don't want you to waste your time, or me to waste mine."

SC: "Couldn't I just put the question?"

AC: "Could we meet to talk about it? The answer is no."

SC: "Perhaps I could just tell you a bit about the project and why I think talking to you would be worthwhile . . ."

AC: "No. Thank you very much, goodbye." (Hangs up.)

This seemed like a minor set-back until we began to explore the ramifications of these exchanges. All requests to interview civil servants who had any connection with or knowledge of the dossier or the Kelly affair were greeted with a curt no. All official requests to the MoD to interview military personnel were denied.

This was particularly surprising, given that I had previously made films about the wars in the Falklands and Bosnia with the full cooperation of the MoD. Even a request to the first sea lord, whom we had interviewed when he was still a humble sea captain, was politely declined. All requests to film in or near any government building were denied, as were requests to film on any Ministry of Defence property, including in any military vehicles or aircraft.

This made many of the sequences we were planning effectively impossible to achieve. We eventually tried privately owned aircraft museums, but even those with the most tenuous links to the military were surprisingly reluctant to help. Of course, all who make drama are used to knock-backs when it comes to securing locations. But this was something different a comprehensively enforced, total ban on any cooperation whatsoever.

For me, the government's attitude to our little drama was summed up by one incident. A scene in our film required a military band. We scoured the country, trying to identify a band that would be prepared to appear. In every case we were rebuffed.

Then, when we were on the point of cutting the scene, we heard back from the Normandy Band of the Queen's Division, based at Catterick. For some reason, London's writ seemed not quite to run in North Yorkshire. The officer in charge spoke to us on a number of occasions and was very keen for his band to appear. We began to plan for their arrival and to accommodate them in our filming schedule. That's when we received the call. The officer had himself received a phone call and would now, unfortunately, have to withdraw. He cited the subject matter. He didn't sound happy at all.

Of course, the programme got made. Simon and his team of researchers spent 18 months finding those, at home and abroad, who were prepared to talk to us. Many had known Kelly personally. Others were involved in the sequence of events that led to the decision to go to war in Iraq and felt strongly enough about the issues raised to speak to us despite the government's strictures.

There were some 120 interviewees in all, to whom I would like to pay tribute, particularly those who agreed to meet us at considerable professional risk to themselves. And I'd like to be able to say that the government's attempt to prevent an examination of this subject in no way limited the programme we sought to make. But, of course, that would not be true.

The causes of Kelly's death are complex and our film attempts to unravel some of them. Whatever may have gone before, the government behaved honourably in the aftermath of his death, setting up an inquiry that was as publicly accessible as it could be. Though some are uncomfortable with his findings, none can doubt that an eminent judge exonerated the government from blame for Kelly's death. So why are they so defensive?

Is it that, for all their "freedom of information" posturing, this is a profoundly secretive government, alarmed by the concept of scrutiny, peopled by lawyers and yet shy of due process, incapable of allowing information to be promulgated without first having the chance to spin it? Or is it that, like all of us, it was troubled by the sight of an old man in a noisy committee room, browbeaten into the service of a dishonourable cause, and feared the effect that the resurrection of that image might have on the eve of an election?

The Government Inspector is on Channel 4 tonight at 9pm