The Crown was allowed to prosecute Angus Sinclair again on the basis that they had new evidence never used before.

This, critics suggest, was the very same evidence that they had failed to produce in his first trial in 2007: DNA.

Critics, not least the late Margo Macdonald MSP, said prosecutors should have told the jury that Sinclair's DNA had been found on tights used to strangle Ms Eadie.

The Crown countered that the DNA was "low copy and low probability". Seven years on and it was DNA on ligatures used to kill the women, including their own underwear, that proved decisive in both securing the new trial and convicting Sinclair.

Forensic scientists, using state-of-the-art white-light technology developed since 2007, were able to identify spots of DNA on items of clothing stored for more than three decades. They included a full profile of Sinclair, trapped in the knots of one of the victim's bras used as a ligature.

In crucial evidence during the trial Geraldine Davidson, a forensic scientist, said the genetic material recovered was 20 million times more likely the DNA came from a combination of Sinclair and his brother-in-law Gordon Hamilton than from either of them acting alone or from neither of them.

Ms Davidson said there had been particular interest in the knotted areas of the tights and bra used as ligatures on Christine, as the knots would "preserve" the DNA present at the time they were tied. She added: "This fits with the DNA within the preserved areas having been present at the time the ligatures were tied."

In the case of Christine's bra, recovered with part of her tights from around her head and neck, Ms Davidson said specialist analysis had been carried out.

This confirmed Hamilton had been the major contributor of DNA within the knots.

But although Sinclair had been the minor contributor in terms of the amount of material left on the ligature, he was "fully represented". She added: "It's a full profile. Some of the components of his DNA are shared with the other three reference samples [Hamilton and both victims], but every one of the components of Angus Sinclair's profile are represented in the profile."

The knots were not just crucial because of the DNA they contained. They also showed, experts said, that two people had tied up the two women.

Lord Advocate Frank Mulholland, prosecuting, had had some key pointers to winning his case from the QC who defended Sinclair back in 2007.

Edgar Prais, when securing Lord Clarke's 2007 dismissal, told the court: "If there had been evidence of Sinclair having tied up the girls, for example DNA evidence on ligatures, I don't think I would be standing here."