TV viewers were given a behind-the-scenes look at how schoolgirl murderer Colin Ash-Smith was finally brought to justice almost 22 years after his horrific crime.

In the final episode of the three-part BBC4 documentary The Prosecutors, police and legal teams were shown building the complex and challenging case against the former milkman which led to his conviction and life sentence.

Ash-Smith, who lived in Swanscombe, was always suspected of the frenzied stabbing of Dartford Grammar School pupil Claire Tiltman in an alleyway in Greenhithe in January 1993, especially after he was jailed for life for two similar knife attacks on lone women in the area.

But it was not until a crucial change in the law which allowed a defendant's previous convictions to be heard by jurors - known as bad character, as well as a confession by Ash-Smith to another prison inmate about how he "snapped and attacked", that prosecutors felt they had enough to charge the 47-year-old with the 16-year-old's murder.

His five-week trial at Inner London Crown Court in Southwark in 2014 was followed by Gold Star directors and producers Sara Hardy and Blue Ryan in their ground-breaking documentary.

The women were allowed unprecedented access to the Crown Prosecution Service (CPS) for the first time in its 29-year history and during filming of the Ash-Smith case they became close to the women who set up the Justice for Claire campaign, four of whom attended every day of the trial.

The childhood friends of Claire, who lived in Woodward Terrace, Greenhithe, also took part in the documentary.

Lisa Gribbin, 38, who was Claire’s best friend at school and also a trial prosecution witness, said: “I didn’t want to watch it at first but it was fantastic.

“It was quite weird sitting outside the box, so to speak, and watching what was going on. A couple of parts were very difficult to view but it was so good – by the end I was in tears. I thoroughly recommend people watch it.”

Other cases featured in The Prosecutors have included a death by careless driving, allegations of indecent assaults and the prosecution of an organised gang who targeted cash machines.

Ms Ryan said: “Until you are the victim of a criminal action directly, the work of the CPS – how they operate and the process involved – is not necessarily understood.

“The professionals and contributors in this series, through their honesty and first-hand testimony, allow the viewers to really understand the work of the CPS, what is involved in their attempt to bring people to account and their relationship with the public.”

At the time of the trial, the CPS defended its decision not to charge Ash-Smith earlier.

It insisted that the change in the law on bad character as well as unravelling the complexities of the case were vital to ensuring the strongest possible evidence was presented to the jury.

Prosecutors looked at the similarities in Ash-Smith’s offending - he attacked two women in 1988 and 1995 - and general character to help identify him as Claire’s killer.

The programme also showed footage of Ash-Smith being interviewed by police after his arrest for the 1995 attack, but in which he denied being Claire's killer.

“Bad character isn’t just about previous convictions,” said Nigel Pilkington, head of the CPS's South East Complex Case Unit who appeared in the episode.

“It is also about other aspects of character such as his carrying of knives and sexual fantasies.

“We were able to see how likely it was that an attack of almost exactly a similar nature in 1995 had occurred two years earlier.

“There are about 100 murders a year in the UK and almost all are committed by people who know each other. Stranger murders tend to have a motive, whether sexual, robbery or theft.

“Motiveless attacks are extremely rare. So how likely was it that this attack in 1993 was replicated years later? This proves identity.”

Ash-Smith was first jailed for life in 1996, with a minimum term of 15 years.

He was charged with Claire’s murder in February 2014 - on the day he was due before the Parole Board.

The original investigation in 1993 was named Operation Artist and was one of the largest Kent Police has ever run.

More than 1,500 statements were taken, there were 40,000 documents and about 20,000 people on the database.

Det Supt Rob Vinson of the Kent and Essex Serious Crime Directorate, who undertook the cold case review in 2013, described the case as "complex, challenging and very tight".

“We took a long time to make the decision (to charge), a number of people were involved and a number of discussions were held," he said. "There are little pieces of evidence which make a difference."

He added: "It is one that has come about into a detailed case – and the devil will be caught in the detail."

Last year, Ash-Smith's appeal bid was rejected by three of the country's top judges and he will have to serve a minimum tariff of 21 years before becoming eligible for parole.