Julie and Graham Hillsdon watched as Angela Dublin was jailed for killing their son in the Eastern Bypass crash, but they are still tormented by many unanswered questions.

The circumstances surrounding the death of their 21-year-old son in last May's tragedy remain a mystery and, four days after Dublin was sentenced to two years' jail, the Hillsdons revealed their anguish.

Only in court did they discover their son was blameless in the crash and never saw the overloaded Citroen Xsara plough across the central reservation towards him.

Dublin's guilty plea means further facts about the crash will never be publicly revealed and there will be no coroner's inquests into the death of Howard and teenagers Josh Bartlett, Liam Hastings and Marshall Haynes.

Mr Hillsdon said: "We want to know exactly what happened on that night. As she pleaded guilty we aren't going to go through a court case and hear every little detail."

His wife Julie added: "In court we heard Howard knew nothing about the car coming across the road through the tall grass. It was the first time we were told he wasn't to blame and was only pulling away from the traffic lights.

"We've heard from different people that he was in the car for a long time, he was OK and unconscious and a friend saw Howard covered in flowers he had bought for a meal. But no one has told us the true situation and we want to know."

Most of the information they have about their son's death was what came out in court.

On the day Howard left their Yarnton home never to return, Mrs Hillsdon said the family shared a four-way hug, including sister Anna, before he left with friend Daniel Orme to cook meals for their girlfriends. Later that evening, they received a phone call explaining Howard had an accident and they thought he'd burnt himself cooking.

They reached John Radcliffe Hospital to discover chaotic scenes.

Mrs Hillsdon said: "I said to a policeman 'I'm Howard Hillsdon's mum'. The police looked at each other and I knew it was serious."

They were taken into a room and told the devastating news.

Mr Hillsdon said: "As a father it's something you never believe you will ever be told in your whole life."

Liam Hastings, Josh Bartlett and Marshall Haynes, all 13 years old, were also killed.

Oxford Crown Court heard Dublin, of London Road, in Headington, squeezed seven children into her five-seater car with only one child wearing a seat belt before crashing into Howard's Honda Civic.

Since the four deaths a £700,000 crash barrier has been installed.

Mrs Hillsdon added: "I can understand the anger of the other families because when you put children into someone else's care you expect them to be cared for.

"She knowingly put those children and Howard into a dangerous environment. It only costs £3.50 for a taxi."

Dublin, 45, admitted four counts of causing death by dangerous driving and was sentenced to two years and banned from driving for six years.