Simon McWilliam's two previous wives described him as a Jekyll and Hyde character, charming one moment, violent and cruel the next.

One said he once grabbed an infant girl round the throat and attempted to shake her.

Both were horrified at the idea of him adopting children but neither was contacted by social services when McWilliam and his wife Michelle applied to adopt.

None of this information was presented to the jury during the trial for fear of prejudice.

However, the jury did hear Michelle McWilliam claiming John Smith would hit and kick her, bruising her face and once splitting her lip. Police privately suspect McWilliam was responsible.

They do not accept a four-year-old boy, small for his age, weighing slightly more than two stone and with no history of violent outbursts, was likely to have been capable of inflicting such wounds.

In court, the McWilliams admitted concocting a lie to police after John's death. Did Michelle McWilliam also lie about John's behaviour in fear of McWilliam or the consequences of conviction for cruelty, or both?

McWilliam had never been in trouble with the police before but his wife had previous convictions for five offences, one for shoplifting in 1987 when she was at college.

Six years later, she was in court for reporting the theft of her cheque book and cheque card, then using them.

Her marriage to McWilliam in 1994 was her first but McWilliam was married twice before.

A son born to his first wife died at the age of three months in 1983, without ever leaving hospital.

He claimed the marriage did not survive the bereavement yet it lasted a further five years.

His second marriage was "on the rebound", had been "wrong from the word go" and was short-lived.

McWilliam began a relationship with Michelle a year before leaving his second wife. He and the then-Michelle Gillard met while working in a factory where he was a manager and she an assembly worker.

McWilliam had two other names, Hall, and a third which cannot be published for legal reasons.

He and Michelle changed their names when they married, "McWilliam" being the surname of Michelle's birth father. He took it as a wedding gift from his wife.

McWilliam was born in Cuckfield and lived in Hove until he was 12, attending boarding school until he was 19. At the time, his family lived in Bexhill.

He returned to live in Brighton in 1984 and, after school, joined the Royal Navy for a short time. He later trained as a draughtsman in Bexhill and worked in pubs in Rye and Reigate before joining an engineering firm in Brighton as a quality assurance manager.

McWilliam felt he had reached a ceiling with his job and decided to take an accountancy degree course at Brighton University where he was described by fellow students as hard-working and diligent.

His previous wives characterised him as capable of both persuasive charm and uncontrolled outbursts of abuse and violence.

Both attributed the breakdown of their relationships with him to his violent personality and cruel behaviour.

Michelle McWilliam had a supportive family in Sussex and relatives testified she was good with children.

She was happy to look after other children and it pained her when she discovered she was incapable of having any of her own. She had an operation in November, 1993, a year before marrying McWilliam, and she twice underwent IVF treatment which failed.

The McWilliams began looking at adoption and turned down two children before accepting John and another child. Social services and the adoption panel were so pleased with the couple that they allowed them to take a third child. Both surviving children were taken back into care after John's death.

John and the second child, who cannot be named for legal reasons, settled well with the McWilliams at first. There is little doubt the couple loved and cared for the children but police suspect a large, instant family proved too much for them.

McWilliam was engrossed in his university studies and he admitted not being a "mummy-daddy", someone who would deal with the messy side of rearing children. Michelle McWilliam, for the majority of time, was taking in contract electrical assembly work at home.

John found himself under a stricter regime from the relaxed atmosphere he enjoyed for 17 months with previous foster parents.

The foster parents remembered seeing John wearing slippers in the McWilliam home, something he was allowed to please himself about at their house. John was also made to change clothes every day after school.

John was a normal four-year-old and not perfect. He was a very slow eater and would sometimes spit out food or vomit at the meal table. He would also occasionally wet the bed.

McWilliam had his own ideas of how children should behave and none of John's problems fitted his concepts.

John did not demonstrate enough love for the McWilliams and they complained when he showed affection for others.

Michelle McWilliam said she wished John would cuddle and kiss her like he did with one of his classroom assistants.

McWilliam admitted smacking the child although, he insisted, never hard, but police believe the cycle of violence worsened until the moment McWilliam or Michelle snapped and unleashed a brutal assault that led to the boy's death.

The Crown's case was not based on that one final assault because it had no proof who was responsible. It prosecuted on the basis of a period of abuse, that either McWilliam or his wife carried out assaults and the other failed to intervene or prevent the violence.

Simon McWilliam spent four months in a mental hospital after John's death and Michelle had a nervous breakdown.

Relatives were in court to support the couple.

The grief was worse for John's birth family and his foster parents who loved him dearly.

For the McWilliams, and those who could have saved him, the guilt will last a lifetime.