I once remarked to a Guardian reporter that what was missing from my long career was a fight against a libel action. That reporter, who had just won a libel case, looked shocked and advised: "Don't even think that way. A libel writ eats at you. It consumes you." I was unconvinced. But now, after many sleeplesss nights in the four years since receiving a writ and despite being vindicated in the appeal court last week, I must admit he was right.

It was not just that being under relentless cross-examination at the high court by one of the country's best-known QCs for three days was one of the worst times of my life. The first warning shot of trouble came six years ago while I was still a BBC home affairs correspondent.

In a court report for radio, I had outlined how a criminal fraudster had told anti-corruption detectives that he had bunged a £50,000 bribe to two Metropolitan police officers, detective inspector John Redgrave and detective constable Michael Charman, a claim he later withdrew. Threats from Police Federation lawyers to the BBC following the broadcast came to nothing. But a year later, after I had become a freelance, officers under suspicion learned I was about to have a book published called Bent Coppers - the inside story of Scotland Yard's battle against police corruption, in which the Redgrave and Charman story featured heavily.

Corrupt officers

With worries about running up huge legal costs, the publishers dropped the book, just as it was about to be printed. However, it was picked up by a bigger company, Orion, who decided to go ahead notwithstanding possible legal problems. They recognised the importance of documenting the story of how the Met was running into difficulties in trying to purge itself of corrupt officers.

Published in hardback in 2003, it received good reviews, and it was not until just after the paperback went on sale that Charman weighed in with a writ, naming Orion and me personally. We learned that Charman was being backed by the Police Federation. Since losing massively in 1997 in a police corruption libel case against the Guardian, the federation was believed to be taking great care before supporting potentially costly libel actions. Why it should be backing Charman, an officer "required to resign" from the Met for discreditable conduct, remains a mystery.

If they thought the publishers would cave in and quickly pay damages and costs, they were mistaken. Orion and its insurers, Hiscox, decided to support me and stand up to the bullying, clunking fist of the Police Federation. Exposing police corruption is obviously in the public interest, and it was on those grounds that we would fight, using what has become known as the Reynolds defence.

Named after Ireland's former premier Albert Reynolds, who unsuccessfully sued the Times, the Reynolds defence of qualified privilege allows publication of unproven allegations that are in the public interest, provided that it is done in a balanced, responsible way.

Orion and I had an excellent legal team, led by Adrienne Page QC from the "defamation chambers of the year", and solicitor Caroline Kean, of Wiggin, a libel expert. Although we believed that Bent Coppers had met the various conditions laid down by the Reynolds defence, proving it was a different matter.

At a preliminary hearing, the trial judge, Mr Justice Gray, decided that because the case involved a book and was complex, he alone would decide it rather than a jury - even though it involved public interest issues. I had gathered a lot of information about Charman, who had refused to give an account of his actions to anti-corruption officers. His cross-examination under oath in the high court would have been very interesting. But it was not to be.

Under Reynolds, it is not necessary for the claimant to give evidence, and Charman chose not to do so. I had three very senior police officers and a respected lawyer lined up to give evidence on my behalf. But they were not allowed into the witness box. It was what was in my mind at the time of writing the book that was important. I was to be the only witness in a week-long case. Nothing could have prepared me for that ordeal.

For no less than three days I was under intense cross-examination by Charman's QC, Hugh Tomlinson, who had successfully represented Prince Charles in a privacy case. He seemed to take me through every sentence I had written about Charman in Bent Coppers and in my 80-page witness statement, and in various other documents and notebooks. I had thought I was on top of the subject, but after an hour or two I was faltering. The alleged bung had taken place 15 years earlier. The court case involving it was in 2001, and I had written the book two years later. The main handicap was that I could use nothing of what I had learned about Charman after I had written the book.

Tomlinson's tactics rattled me. I felt ambushed when he went into areas over which he was supposed to have given prior warning, but had not done so. There was also a huge amount of documentation. Tomlinson dodged from one to another, without giving me time to check files.

I decided to take advice from my legal team, but it was then that the advice given to all court witnesses sank home. I could not talk to anyone about any aspect of the case while I was still giving evidence. My lawyers would not even tell me if I was doing OK. I have never felt more alone than over the two days of questioning that followed.

Reflected badly

I did not disagree when Tomlinson said I had set out to tell "a rollicking good story of police corruption", nor when he said that an author should include balancing material when dealing with potentially defamatory allegations. But I took issue when he claimed that I had not done enough. In fact, I had deliberately left material out of the book which reflected badly on Charman.

Mr Justice Gray's judgment was a shock. He seemed to side with Tomlinson on everything. Dismissing my side's attempt to have Charman's libel action dismissed, he ruled that Bent Coppers did not pass all the necessary tests of "responsible journalism" and that I was not entitled to protection under the Reynolds defence.

Mounting an appeal was a time-consuming and expensive affair. But our lawyers believed that Gray's judgment contained basic mistakes and there was a wider issue too, affecting all publishers of current affairs books - that authors were not subject to newspaper time constraints, so it was incumbent on them that they should do more checking and verifying.

A House of Lords ruling last autumn in the Wall Street Journal and Mohammed Jameel case provided encouragement. Coming after Gray's judgment against us, the Law Lords said that libel judges in the lower courts had been interpreting the Reynolds defence too strictly. They called for a greater relaxation. We decided to appeal. The appeal took place over four days last March, but it was not until last Thursday that the three appeal judges ruled. They said that Gray had "erred", and some of his criticism of me was misplaced. Overturning his judgment, they concluded that I had behaved as an honest, responsible journalist.

Although I was vindicated, one aspect of this affair will always rankle. Fighting this case as a freelance journalist has taken up a great deal of time over the past few years. There is surely something wrong with a system where, if he had won, Charman would have been awarded tens of thousands of pounds, whereas I, found to be an honest responsible journalist, will receive nothing in damages.

This has certainly been an experience, but one which I would never wish on any other journalist or author.