Sion Jenkins did not have the motive or the opportunity for killing his foster daughter, lawyers told his appeal hearing yesterday.

But, they say, he did have an alibi, which jurors at his 1998 trial never heard.

On the first day of a three-week hearing at the Court of Appeal, his lawyer Clare Montgomery attempted to demolish the case against Jenkins and in its place suggested Billie-Jo was murdered by a loner with a plastic fixation.

She described the prosecution case as "not merely improbable but wrong."

And she said Jenkins had been the victim of a terrible injustice, adding: "He has not only suffered the tragic loss of a child through murder but also been wrongly convicted of that murder."

Jenkins, 46, was jailed for life for bludgeoning 13-year-old Billie-Jo to death with an 18in metal tent spike as she painted a patio door at their home in Hastings in February 1997.

The jury at his Lewes Crown Court trial was told that after battering her he went shopping at a local DIY store with his natural daughters Lottie and Annie, then pretended to discover Billie-Jo's body when he returned.

There were more than 150 microscopic spots of the teenager's blood on his jacket, which were consistent only with his having been the attacker, the trial was told.

In December 1999 the Court of Appeal rejected Jenkins' initial appeal.

But in April 2001, a file containing new evidence was handed to the Criminal Cases Review Commission and a fresh appeal was ordered.

The appeal judges will examine fresh expert evidence relating to the blood spots, which Jenkins claims must have been spattered as he attended the dying girl and a bubble of blood burst in her nose.

They will also examine evidence that a mentally ill man, referred to as Mr X, who was known to have been in the vicinity when Billie-Jo was killed, had shown signs of having a fixation with pushing pieces of plastic bag up his nose.

A pathologist found that Billy-Jo had part of a black bin liner stuffed deeply into one of her nostrils.

Miss Montgomery said that in Jenkins' trial it was alleged that during a three-minute visit to the family home on February 15, 1997, he flew into a rage with Billie-Jo.

He was accused of walking past her on the patio - ignoring several potential weapons, including a hammer, on the patio table and taking a tent peg from a fuel bunker near the side gate.

Then, it was claimed, he battered her more than ten times, bending the peg, and pushing part of a plastic bag into her nose before leaving with his daughters Lottie and Annie on a shopping trip with no sign of blood or paint on him.

Miss Montgomery said there was no evidence of any incident that could have triggered such a "murderous rage".

She said the far more likely explanation was that Billie-Jo was killed by an intruder who entered through the side gate near where the tent peg was stored. The court heard the prosecution secured a conviction using evidence from forensic scientists who said the 150 microscopic spots of blood found on Jenkins' clothes could only have been produced during a frenzied attack on Billie-Jo.

Miss Montgomery said new evidence would show the blood spots were more likely to have been caused by blood exhaled from the girl's airways as he leant across her stricken body.

She said none of the scientists who said the blood on Jenkins showed he must have been the murderer had "sufficient training, skill or knowledge" to state that conclusion with anything like certainty.

She claimed this critical issue was determined "in almost complete ignorance" of the necessary scientific material.

The evidence showed that Billie-Jo's airways were blocked at some upper point under fantastically high pressure and any release of that blockage would be one of air moving at a very high speed.

There was absolutely no need for Billie-Jo to have been alive or breathing for Sion Jenkins to become covered in her blood.

A bubble of blood bursting in Jenkins' presence from Billie-Jo's nose could have caused that spatter.

At Jenkins' trial his defence counsel, Anthony Scrivener QC did not call Lottie and Annie, who were with him when he discovered the body, as witnesses.

This was despite their accounts originally suggesting Billie-Jo had been alive when they left the house after the brief visit and their father came out almost immediately afterwards, giving him little time to commit the crime.

But it was believed the daughters had become hostile to their father.

Over the course of the year before the trial, their mother Lois Jenkins told police she had had conversations with the girls and their accounts of the day were significantly changing.

As a result of police reports about their girls' conflicting evidence, Mr Scrivener decided not to call them as defence witnesses because they were no longer seen as helpful.

Miss Montgomery said Lottie and Annie, who were with Jenkins when he discovered the body, had given accounts to the police which effectively precluded his being the murderer.

She said: "Indeed, their accounts strongly suggested that the murder had been committed by an intruder who had gained access to their home by the back gate."

The intruder theory was supported by Lottie, who told police the side gate was closed when they left but open when they returned from the DIY store.

If Billie-Jo had been alive when they left the house, Jenkins would have had no time to kill her.

Miss Montgomery said the evidence from the two girls made it almost impossible for their father to have been the murderer.

She said fresh evidence from the two girls, which would be presented in his appeal, confirmed their original accounts "and establishes that their apparent hostility was the product of their mother giving inaccurate information to the police."

The prosecution's view was that Lois Jenkins could have had no reason for giving such misleading information.

Miss Montgomery said the mother was told by police that her husband was undoubtedly guilty and she believed it to be so.

She said: "So you have a mother who believes her husband, the father of her children, has killed one of them.

"She is terrified about him returning home and she understands that the children's evidence nevertheless might lead to her murderous and dangerous husband being released and being sent back into the family.

"Any mother faced with that prospect would try to unpick the children's stories.

"It's an impossibly difficult situation and for the prosecution to say that Lois Jenkins had no reason to lie - she had every pressing and decent reason to lie if the information the police had given her was accurate."

The court heard Mr X was a schizophrenic who lived near the murder scene and was seen behaving strangely in the area on the afternoon of the murder.

Miss Montgomery said that after being arrested on suspicion of the murder, Mr X was seen by police officers trying to put pieces of plastic bag into his mouth and nose.

Mr X was eliminated as a suspect when his clothes tested negative for Billie-Jo's blood.

But the Criminal Cases Review Commission, which referred the case back to the Court of Appeal after detailed re-investigation of the evidence, had concluded that Mr X did not have a complete alibi for the afternoon of the murder.

It was also not clear whether police seized all the clothing he was wearing that day.

Miss Montgomery said the prosecution's case was "thin" and always depended on evidence that could have had an innocent explanation.

The conviction of Sion Jenkins was unsafe and should be quashed, she said.

Referring again to the evidence about the blood spots, she said: "This was a scientific hinterland.

"No one knows now where the truth lies in this case but what is certain is that the jury was wholly misled that they could be confident that the blood could only have got there one way - that namely Sion Jenkins was the murderer.

"In my submission, that certainty was wholly misplaced."

The hearing continues.