Ivan Fallon: The Vanunu I knew was naive, confused, an easy target for a Mossad honey trap

His story blew the lid on Israel's secret nuclear arsenal. But the man Ivan Fallon met in 1986 cut a very different figure from the one now revered and reviled as hero and traitor

Watching Mordechai Vanunu emerge from his Israeli jail last week, I felt a surge of admiration for the man. Eighteen years ago he sat outside my office at The Sunday Times in Wapping, a diffident, frightened, but above all - and this was the abiding impression of all of us after we'd lost him - naive man, with only a half-formed idea of what he was doing or why he was doing it.

Watching Mordechai Vanunu emerge from his Israeli jail last week, I felt a surge of admiration for the man. Eighteen years ago he sat outside my office at The Sunday Times in Wapping, a diffident, frightened, but above all - and this was the abiding impression of all of us after we'd lost him - naive man, with only a half-formed idea of what he was doing or why he was doing it.

The figure who stepped out of Ashkelon's Shikma high-security jail on Wednesday was greyer, his hair a bit thinner, but otherwise remarkably unchanged, the years of isolation and obloquy lying remarkably lightly on his still unfurrowed brow. But this was a very different Vanunu, a maturer, stronger, far more contained man, proud of what he had done and eager to have his day in the sun.

He held his fingers aloft in a double V-for-victory sign, and announced that the Israeli security services had not broken him, nor driven him crazy. Very clearly they had not - he seemed saner and certainly an awful lot wiser than I remembered him.

His reappearance contrasted starkly with his disappearance, spirited away by Mossad back in 1986 as he was about to provide the first real evidence that Israel possessed a highly sophisticated nuclear arsenal ranking only behind the four major powers. The same media that had ignored his revelations was now waiting for him in phalanxes.

Vanunu, at nearly 50, has began a new chapter in what up to now has been a bizarre and contradictory life, full of self-doubt, introspection and remarkable inhibition. When we brought him to London in 1986 to check out his story, it soon became apparent that sexually he was still a virgin at 31 and desperate to change that status (18 years in prison will presumably not have given him much opportunity since). The bright lights of London's West End and Soho held him in thrall, and it was an apparently chance encounter with a pretty blonde woman (whom we later named Cindy) in Leicester Square that led to his abduction.

Much has been written over the past week about his motives, his conversion to Christianity and about the secrets he can still reveal. Let me try to put some of it into the perspective of that defining month of September 18 years ago, when Vanunu's fate was decided and when the world, whether it wanted to believe it or not, became fully aware of the extent of Israel's nuclear armoury.

The story - or at least this part of it - starts in Australia, where Vanunu had ended up after a wandering tour that had taken him out of Israel and across Asia. He had travelled through Nepal, Burma and Thailand with no determined goal or destination, letting his fate take him where it wanted to. In his backpack he still had his camera and two undeveloped films which he had used in his last days at Dimona to record secretly the inner workings of the environment in which he had worked for nine years as a nuclear technician.

Until he turned 30 there had been nothing in his background to give any hint of where his journey in life would lead him. In his early years he was a religious Jew, did his stint (as a sergeant) in the Israeli army and was bright enough to be sent to Dimona as a technician (but not clever enough to make it to the "nuclear scientist" layer which he coveted). It was only after some years there that the doubts began to set in, and he fell in with a group of dissidents, arousing the suspicions of the security forces. Faced with redundancy, he smuggled his camera into his workplace, took his photographs (without having any idea what he would do with them), collected his $7,500 payoff and set off. In Sydney he took any odd job he could get and, in this still disoriented state of mind, strayed upon his local Christian church, St John's, where the Rev John McKnight welcomed and later converted him.

It was while painting the church walls that he met Oscar Guerrero, a freelance journalist from Colombia who was down on his luck. Guerrero teased his story out of him and persuaded him he owed it to the world to make it public - only Vanunu, he argued, could prevent a Nagasaki in the Middle East.

The next stage was a farce. The Australian media, approached by Guerrero with a reluctant Vanunu in tow, simply didn't want to know. Nor did the local correspondent for Time magazine. Then someone mentioned the London Sunday Times, where the story was passed up the line until it reached Robin Morgan, then head of the Insight team. Morgan took it to me in my capacity as acting editor (Andrew Neil, the editor, was launching Sky Television for Rupert Murdoch at the time, but became deeply involved in the story in the later stages), and I agreed to send Peter Hounam, a bearded investigative reporter, to Sydney to check it out. A few days later he reported back: he had found some holes, but it stood up.

And so Mordechai came into our lives, stepping gingerly through the pickets around Wapping and entering an office that was deeply sceptical of his story. The Sunday Times was still haunted by the fake Hitler Diaries episode six years earlier, which had cost the then editor his job, and this was potentially in the same league. Was Vanunu in reality a Mossad spy? An Arab plant? A con man? Or for real?

Morgan mustered his team - about 12 people in all, including Professor Frank Barnaby, late of Aldermaston and Harwell and one of the country's leading nuclear experts. Checking the story was tedious and frustrating work, and Vanunu was bled dry of every scintilla of information he possessed. Last week a spokesman for the Israeli Ministry of Defence insisted "he still possesses state secrets, including some which he has not revealed". It is nonsense: it is now nearly 20 years since Vanunu worked in Dimona and 18 since he divulged everything.

Publication day was set for the end of September 1986, but at the last moment there was a hitch. Guerrero, kept out of the loop by everyone, including Vanunu, arrived in London and placed a spoiler story with the Sunday Mirror, probably - we were never sure of this - as a result of an Israeli request to Robert Maxwell, the then owner. It was a very twisted version of events and was designed to discredit Vanunu.

A few days later there was an even bigger disaster which completely blindsided us: we lost Vanunu. He went awol one evening, slipping his Sunday Times shadow in Leicester Square to go off with his new blonde friend, Cindy. He was pretty edgy by that stage, badly shaken by the Mirror spoiler and by the fact that the Israelis now knew he was in London. He was also scared of what was about to happen to him, knowing he would be a marked man for the rest of his life. And he was desperate to get laid.

On publication day he was still missing, and the paper was faced with its worst dilemma for years: should it pull the story or go ahead without its one and only witness? In the heated debates that day, the senior editorial staff were pretty evenly divided between those who were now convinced he was a spy or a plant, and those who believed he was for real. Finally, Neil, now very heavily involved, took the decision: go with the story.

It was weeks before we knew what had happened to Vanunu. He suddenly appeared at the window of a police van in Tel Aviv and squeezed the palm of his hand against the grid. He had written the words "Hijacked in Rome, 30/9/86" and the number of the British Airways flight from London which he had shared with Cindy. He was charged with treason, sentenced to 18 years in jail - and served every day of it. I'm delighted he is out.

Ivan Fallon is chief executive officer of Independent News & Media UK