Who was the real Dr Kelly? Innocent or serial leaker, honest victim or liar, scientist or spy?

The Hutton inquiry into the events resulting from that conversation may never be able to discover what was going through Dr Kelly’s mind on his way to the woods two weeks ago.

But Lord Hutton will at the very least seek to establish exactly who Dr Kelly was, because the answer is not as obvious as it might seem.

Different accounts in past weeks have suggested that he was a somebody and a nobody; a victim of the BBC and a victim of the Government; an honourable man and a liar; a media innocent and a serial leaker; a scientific technician and a spy.

What everyone can agree on is that Dr Kelly was a husband and father of three, a former Porton Down scientist and UN weapons inspector in Iraq, who had spent the past four years working for the Ministry of Defence as an expert on biological warfare at the Proliferation and Arms Control Secretariat.

It is also incontrovertible that on July 9, he was named as the likely source for Mr Gilligan’s allegations that Downing Street had “sexed up” last September’s dossier on Iraq by inserting — against the wishes of the intelligence services — information the Government knew was probably wrong.

The “classic example”, Mr Gilligan quoted his source as saying, was the claim that Iraq could deploy weapons of mass destruction within 45 minutes. He later added that Alastair Campbell was responsible for such changes. Since Dr Kelly’s death, the BBC has confirmed that he was the source for Mr Gilligan, as well as for subsequent reports by Susan Watts and Gavin Hewitt.

How did his name emerge into the public domain? The Government says that the BBC had dropped hints but also concedes privately that Downing Street and the MoD made misjudgments in setting up a process for naming him. The fingerprints on this scheme may include those of Sir Kevin Tebbit, the Permanent Secretary at the MoD, and Geoff Hoon, the Defence Secretary. It involved offering journalists a unique “confirm or deny” service.

Instead, senior Government figures say that they would have been within their rights to go “through the front door” in identifying somone who had breached rules about speaking to the media. But no one can agree on what he said to these journalists and Dr Kelly sometimes appears to have even disagreed with himself.

When he gave evidence to the Foreign Affairs Select Committee three days before his suicide, he convinced MPs that he was not Mr Gilligan’s source. Afterwards he was said to be upset, possibly because his testimony contrasted with what he had said the previous day in a session with the Intelligence and Security Committee. In his cross-examination by the Foreign Affairs Committee, Dr Kelly began by saying: “From the conversation I had with him (Mr Gilligan) I do not see how he could make the authoritative statement he was making.”

Then the witness allowed himself to be led. No, he could not be the main source. Asked about a quotation attributed by the BBC’s Ms Watts, he said: “I find it very difficult. It does not sound like my expression of words.” So, you deny those are your words? “Yes,” he muttered.

But at other times in his evidence, Dr Kelly perhaps had only himself to blame for giving a misleading impression. He told MPs that he had met Ms Watts only once and that he could not have made comments justifying her report “on that occasion”. When asked if he had spoken to Mr Hewitt, he replied: “Not that I’m aware of, no. I’m pretty sure I have not.”

These details did not matter on July 14 because the committee’s initial verdict that Dr Kelly was not the “main source” seemed to have vindicated the BBC. Andrew Marr, its political editor, told the nation on the night of July 15 that the Government had thought it was “playing its ace” by disclosing Dr Kelly’s name but the suspected source had “turned out to be a joker”.

But for four days between publication of Dr Kelly’s name and his evidence to MPs, the BBC had imposed a virtual news blackout on him. Its sensitivity was not rooted in the high journalistic principle of protecting a source because Dr Kelly had already admitted he had met Mr Gilligan.

Instead, the corporation was trying to protect its credibility. The BBC’s problem was that Dr Kelly did not fit Mr Gilligan’s description. He was not “one of the senior officials in charge of drawing up the September dossier”. Nor was he a “senior and credible source in the intelligence services” as claimed by Richard Sambrook, the head of BBC news, who had long known Dr Kelly’s identity.

According to the MoD, Dr Kelly played no part in the dossier other than providing some input for a background section on UN weapons inspections during May or June last year. He was not a member of the intelligence services, had not seen the key material relating to the 45-minute claim, and was not in a position to know if Downing Street or Mr Campbell had wanted to “sex up” the document.

Since Dr Kelly’s suicide and the confirmation that he had been the source, the BBC has changed emphasis. References to the source being in the intelligence services were “slips of the tongue”, says the corporation. But the BBC maintains that Dr Kelly was an “intelligence source” in the broadest sense because he knew a lot about Iraq’s weapons of mass destruction and would have seen secret material.

In the same breath as saying that he was an important and credible source, the BBC is also implying that he may not have been entirely truthful in his evidence to MPs.

The BBC says that the notes made by Mr Gilligan on an electronic organiser at the Charing Cross Hotel are supported by a tape made by Ms Watts. Both quote their source expressing reservations about the 45-minute claim and alleging varying interference.

The scientist does appear to have had strong views about the 45-minute claim, which he believed had confused firing WMD with the time needed to order a deployment.

Dr Kelly is said to have mentioned his concerns about the interpretation of intelligence in the dossier as early as October when he gave fellow followers of the Baha’i religion, which advocates world peace, a slide show in a living room in Abingdon.

But the Baha’is account of that talk, as well as the reports by both Ms Watts and Mr Hewitt following their conversations with Dr Kelly do not repeat Mr Gilligan’s assertion that Downing Street inserted the 45-minute claim knowing it to be wrong. Indeed, a second report by Ms Watts said: “Our source was not disputing that the 45-minute assessment was included in the dossier by the intelligence services.”

It must also be doubtful that Dr Kelly used words such as “sexed up”, a phrase much more to the taste of Mr Gilligan, if his Radio 4 report on the dossier’s publication on September 24 is anything to go by. He described the document as “rather sensibly cautious and measured in tone” before adding: “There are a couple of sexy lines like the fact he (Saddam Hussein) can deploy within 45 minutes if the weapons are ready . . . which we actually knew.”

Eight months later the same journalist was quoting his “intelligence source” saying this “sexy line” had been inserted by Downing Street who knew it was probably unreliable.

The row that ensued has seen both the BBC and the Government seeking to destroy each other’s credibility in a battle to defend their own. The Hutton inquiry into the circumstances of the death has the potential to inflict serious damage on both institutions.

Even on the narrowest interpretation of his remit, Lord Hutton will not just be trying to discover who was telling the truth. He must examine whether the behaviour of Downing Street and/or the BBC led Dr Kelly to take that last lonely walk in the rain up to Harrowdown Hill.