It was during his trial that I realised that someone had to write about "Doctor Fred" and it might as well be me. The highly regarded GP was always known by his homely middle name. Only the judge who formally sentenced him to imprisonment for life called him "Harold Shipman", so that's the name that will be for ever recorded in the record books of murder. "Doctor Death" the headlines dubbed him.

Shipman's crimes will be, of course, for ever devastating to those most directly affected. Nevertheless, I believe that we're all somehow involved in this horror. The revelation of a local doctor's massive abuse of trust sends shock waves through the whole of society, destabilising the unthinking confidence we place in the authority of professionals.

Even when Shipman's guilty conviction for murdering 15 of his faithful patients was announced, some of those who had been on his books continued to maintain that the arrest of this "pillar of the community" was either some inexplicable error or, worse, a deliberate police witch-hunt.

The "factual drama", or whatever the current term of approval or contempt might be, has a long and, for the most part, honourable tradition on British television. But dramatists still find ourselves having to justify why we "resort" to the "manipulative" devices of "fiction" (always a dirty word to those print journalists and documentary-makers who have never, of course, been known to put a spin on truth, or to aspire to have an effect upon their readers or viewers). I've found over the years that those who accuse dramatists of ambulance-chasing are often those who wish they'd been first in the race, so I make no apologies for using drama as a way of coming to terms with what is really happening around us and to depict honestly how the world we live in is thinking about itself.

There are two responsible kinds of "drama-documentary". One attempts to review contentious, problematic moments of our collective public history. The clarity of dramatic depiction is mobilised, in a way that conventional documentary cannot be, in order to expose hidden injustice. The second type, however, tries to shine a beacon on to some apparently well-known case, to delve beyond the surface of newspaper headlines. It needs to pick away at some terrible, aberrant social sore that somehow reveals the workings of the society we've all carded up to.

In both cases, it is the very nature of suspenseful storytelling, the ambiguities of characterisation, the texture of dialogue, that must be mobilised to reveal the emotional impact real events actually have upon those participants caught up in them. Clearly, Shoot to Kill, which I wrote for Yorkshire Television more than a decade ago, was an instance of the former. But Shipman fits into the latter category. It's not a "we name the guilty parties" story.

Of course, there are some very pertinent institutional and organisational questions that remain to be asked so that nothing like this ever happens again. But the main purpose of writing Shipman was not to point a political finger or wallow in the psychopathology of a bizarre and demented individual. And none of us involved in the film ever had the slightest desire to exploit or compound the grief of anyone personally connected to Shipman's many innocent victims, either those he was convicted of killing or those hundreds more for whose murders he will never be charged.

The reason that I wanted so much to dramatise a particular aspect of these terrible events is simple and modest. I came to the conclusion that the story that needed to be told was about the how, what and when, rather than the imponderable why.

When did it look as if vague rumours on the street might be pointing towards a series of horrific crimes? How was a scientifically grounded forensic case meticulously built up against Shipman? What did it feel like for a senior policeman gradually to realise that a respected member of his community might well be a mass murderer, but that he would have to exhume a dozen bodies of his neighbours to prove it?

So it was always my intention to make a film about the investigation rather than a film about Shipman or his victims. It is never easy to write a crime story where every member of the audience knows the ending. But the case against Shipman was very tricky and involved some amazing detective work. It was by no means cut-and-dried until the very moment the jury returned their verdict.

I could not have written the film in this way without the confidence of Detective Inspector Stanley Egerton. Stan had been the Operations Officer for the long, uncertain and complex investigation into Shipman and, as such, was one of the four top detectives who led the team under the overall command of Detective Superintendent Bernard Postles. However, it soon became clear, to me at any rate, that Stan's involvement with the case was central and pivotal.

Egerton had been on holiday when the first complaint against Shipman had been lodged and a far too secret, superficial and hedged-in inquiry had taken place without his knowledge. Why this came to nothing and Shipman was able to kill three more of his patients is one of the issues that the current inquiry will presumably soon make public. But Stan lived in Hyde and knew personally the local people who were beginning to become suspicious about this otherwise much-loved GP.

Stan could not dismiss these suspicions, at this stage entirely anecdotal, as mere idle or malicious gossip. Then he was asked to look into the possible forgery of a former lady mayoress's will, and it was his street-smart copper's instinct, I remain convinced, that initially kept the case alive when the evidence against Shipman was circumstantial at best. He was a few months off retiring: like some overused cliché of the genre, this really was Stan's last case. His dogged need to "think the unthinkable" meant that he was cursed and spat at in the streets by Hyde residents who knew that their "Doctor Fred" was an exemplary medical practitioner.

Shipman is constructed as a drama, but based entirely upon objective truth. Though I must admit that there are some scenes that I simply made up... or so I thought. I had come to the conclusion that I would never get a handle on the psychology of the perpetrator of these despicable crimes. I had listened sceptically to the psychobabble of self-appointed media shrink experts, who were queuing up to explain why this sociopathic doctor made the decision to play God because his mother died when he was teenager, but I was still left with a gaping moral black hole.

The very question I had turned away from earlier - why did Fred Shipman kill? - could not be ducked, even if it could not be answered. I'd discussed this long and hard with Stan Egerton. He had got to know this killer well, but Shipman's motivations still remained as elusive and perplexing to him as they did to me.

I knew that Stan was a religious man and that this was, ultimately, a spiritual problem. So, I eventually wrote up a version of our speculative conversations as if they had been discussions between this real policeman and a putative, fictional vicar.

When we finally got the green light, I was, not unnaturally, rather worried about the importation of my own attempts at metaphysics and insisted Egerton read these scenes I thought I had entirely concocted and to which he might justifiably take exception. The response was prompt and unsettling: it's uncanny. How did I know he had engaged in such conversations? Had I crawled inside his head? I had, of course, got a few details wrong. For example, while Stan really did confide in a vicar, he didn't look much like my version. And what the pair imbibed while Stan was struggling to make sense of Shipman was not whisky, as I wrote, but beer. So, before the accusations come flying, I admit that the inclusion of this particular spirit in the film is a lie. But the content of the discussions was, according to Stan, pretty much what actually took place.

When the retired detective saw the film, there was only one aspect to which he really objected: I had made him into a hero. I had to explain that I usually don't do heroes... but, to tell the truth, Stan Egerton was some kind of a hero to me. Two weeks later, he was dead. And I finally got to meet the vicar... when he buried him.

The detective lies in the cold ground. The doctor sits in a warm cell. I would only wish that this film is factually clear and emotionally honest. This is what Inspector Stan Egerton would have wanted most of all. This is what Shipman would do everything in his power to prevent.

During the course of this lengthy and troubling investigation, when it seemed as if graves were being opened on a weekly basis, local wags reacted to the impossible implications of the whispers that were spreading through macabre humour: "Did you know Spielberg's making a film in Hyde? Shipman's List." "Aye, but Walt Disney's beaten him to it - One Hundred and One Cremations." Some might find this cruel, to me it feels like coping.

A scene I wrote that was, to my regret, never filmed (for reasons of logistics not of taste) was based on an incident told to me by Stan that allowed him a rare moment of respite from quotidian horrors. One Saturday afternoon during the investigation, he went to support his local team, Hyde United. Their striker was brought down, the magic sponge was applied and the player limped back on to the field. DI Egerton couldn't stop himself from guiltily laughing as the opposing supporters began to chant: "One Doctor Shipman... There's only one Doctor Shipman..."

Let's hope they were right.

· Shipman is on ITV1 on July 9 at 9pm.