A mother whose toddler was allegedly crushed by a car seat has been convicted of child cruelty and assaulting a witness, while the boy’s stepfather was found guilty of intimidating a witness in the case.

Three-year-old Alfie Lamb died from crush asphyxia shortly after a car journey with his mother, Adrian Hoare, and her partner, Stephen Waterson, a court heard. The prosecution alleged that the latter crushed the child, who was sitting in the rear footwell at his mother’s feet, by pushing his car seat back.

Facebook Twitter Pinterest Adrian Hoare had previously pleaded guilty to conspiracy to pervert the course of justice. Photograph: Metropolitan police/PA

Waterson, 25, denied that, saying the child, nicknamed “Little Tarzan” by the defendants, must have suffered the injuries by some other means. The jury was unable to decide on a count of manslaughter against him. Hoare, 23, was found not guilty on the same charge.

Hoare and Waterson had previously pleaded guilty to conspiracy to pervert the course of justice.

The three had gone shopping for cushions in Sutton, in south London, on 1 February last year, along with three other people: 19-year-old Emilie Williams, 22-year-old Marcus Lamb and another young child.

The jury at the Old Bailey was shown CCTV footage of Alfie running to keep up with his mother moments before he was put in the car for the journey back to nearby Croydon.

It was alleged that Waterson, a night club worker, got annoyed at Alfie’s crying and twice moved his front passenger seat into him as Alfie sat at his mother’s feet. Jurors heard the maximum space in the foot well was 30cm, which could be reduced to just 9.5cm at the touch of a button.

Waterson, who is 5ft 7in tall, initially pushed his front seat back to give himself more leg room, according to Williams. Alfie screamed for his “mummy”, but she just slapped him and told him to “shut up”, said Lamb, the unqualified driver and Alfie’s cousin.

By the time the group arrived at Waterson’s home in Croydon, Alfie had collapsed and stopped breathing.

As medics desperately tried to revive him, Waterson fled the scene and Hoare lied, claiming she had been in a taxi. Waterson gave officers a false name and false statement and sold the Audi.

Alfie died three days later.

After the incident, Hoare, Williams and Waterson, who is the adopted son of the former Tory minister Nigel Waterson, lied to police about what had happened. Each pleaded guilty to conspiracy to pervert the course of justice, while Hoare was convicted at trial of assaulting a witness and child cruelty. Waterson was convicted of intimidating a witness.

Facebook Twitter Pinterest Stephen Waterson, adopted son of former Tory minister Nigel Waterson, was found guilty of intimidation of a witness. Photograph: Metropolitan police/PA

Waterson was alleged to have threatened to make Hoare and the other witnesses “disappear” if they did not stick with their fake stories.

Hoare eventually broke her silence and told her half sister, Ashleigh Jeffrey, in a taped conversation handed to police. But Waterson blamed Lamb, whom he regarded as a step-brother, for being a “grass” and put his foot on his head during a violent assault in Crystal Palace park that was filmed on his mobile phone.

Jurors were told Waterson was a controlling womaniser who used his family connections to “powerful people” to control and manipulate people.

He also had a violent temper, with three previous convictions for attacking an ex-girlfriend and his sister’s husband.

Giving evidence, he denied he would hurt a child and said he moved his seat back once by up to an inch. It was suggested on his behalf that Hoare must have done something to cause Alfie’s injuries.

The prosecutor Duncan Atkinson QC told jurors both had played a part in Alfie’s death, saying Hoare “fundamentally and fatally” had failed to act.

Atkinson said: “When he started to cry, when he said he did not have enough room, when he coughed as if he was about to be sick, when he was screaming, when she could see he did not have enough room.

“All she needed to do was pick him up and she didn’t and he is dead.”

Responding to the verdict, Scotland Yard’s DCI Simon Harding said: “He was a vulnerable three-and-a-half-year-old who had his life in front of him. He had no one to care for him on that day.”

Harding added: “Adrian Hoare, as a mother, has the same responsibilities as any other mother who is looking after a child, especially of that age as it’s paramount she looks after their safety and their wellbeing. It’s clear from her actions on that day she really did not have concerns.”

Following the verdicts, Mr Justice Kerr discharged the jury. Atkinson asked for seven days for the Crown to decide whether to seek a retrial for Waterson on the manslaughter charge.