"I have come close to losing my mind because the whole thing is close to insanity," Andre Hanscombe, 33, said. "There are still times when I feel enough is enough, that I should go now. But I don't because of Alex."
The child, then two, was found clinging to his mother's body, covered in her blood and too shocked to speak. He rarely cries over his ordeal, said Mr Hanscombe, who was speaking in advance of the publication of his book, The Last Thursday In July: The Story Of Those Left Behind.
"His survival instinct is so strong and he copes so well that you get used to him being almost blase about everything.
"When he does break down it is terrifying. You realise that it is there all the time and he is just dealing with it.
"Maybe once a year something will trigger it off. He nearly lost his thumb in a farm gate once and he went to pieces, shouting, `I'm going to die, I'm going to die'. It was total trauma. All sorts of things came out over that but he was back in control that afternoon."
Alex, the only witness to the murder in July 1992, weeks before his third birthday, describes his mother's killer as "the bad man". He can recall the killer and his clothes, the knife he produced, and how he washed in a stream after his savage attack. But he has never described how his mother was sexually assaulted and stabbed 49 times. "The moment he focuses on that it is overwhelming and he blanks it out," Mr Hanscombe told the Mail on Sunday.
The murder remains unsolved. Colin Stagg, a 33-year-old convicted sex offender, was acquitted of her murder when the judge, Mr Justice Ognall, ruled that the prosecution evidence, obtained by an undercover woman police officer, was inadmissible. Alex has never been shown a photo of Mr Stagg and has never been asked formally to identify him.
Mr Hanscombe, who moved to France with his son shortly after he murder, said he could not look to the future. "There used to be a point I looked to - `after the trial'. But now there is nothing to focus on ahead," he said. Neither could he contemplate having another partner. "Rachel made me promise if anything ever happened to her that I would find somebody else - not become a hermit, living on memories," he said. "I told her the same thing so I know she is right intellectually, but even the thought of it makes me feel unfaithful. I still have a relationship with Rachel and I don't want to let her down. She has been hurt enough as it is."
Mr Hanscombe began writing his book shortly after Ms Nickell's death. "I wrote it so that my memories would be permanently stored and so that I could find some peace," he explained. "This way I am not going over things with the same intensity, afraid that things might slip away from me."
The book is also for Alex, so he can read it "when he is ready". But Mr Hanscombe realises that he may not wish to do so. "Perhaps his curiosity to know more about Rachel's death will not be awakened because he knows enough. He already knows what happened. He saw everything."