The incident was kept off Karl Chapman’s custody record and not disclosed to the Crown Prosecution Service ahead of his giving key evidence in the murder trial.
The failure to reveal Chapman’s access to heroin at Millgarth police station was among a catalogue of police misconduct which eventually led to murder convictions against Paul Maxwell and Danny Mansell being quashed.
Custody Sergeant Peter Haxby told investigators he checked on Chapman after noticing he had retired to his cell unusually early.
Chapman was in a deep sleep and Sgt Haxby recalled that “... on his bed at the side of him was a lighter with a piece of foil with some brown substance on it which I took to be heroin residue.
“The way Chapman was completely out of it, as he was for the rest of the night, that took place an hour or two... after he’d had some clothing brought in which had come from his mother’s.”
Pamela Chapman admitted she had supplied the heroin. Sgt Haxby said he compiled a report but the only result was a warning.
When interviewed, Chapman recalled the warning and said he had joked at the time that it was his “second final chance.”
The investigation report concluded: “All the evidence indicates Karl Chapman was given a warning in connection with this incident and no investigation was made into his possession of the drug or his mother’s supply of it.
“The incident was ‘covered up’ and not disclosed to the prosecuting authority. The decision to take no action could only have been taken to ensure Karl Chapman’s continued co-operation with police investigations and the outstanding trials.
“Mrs Chapman received favourable treatment and was not investigated for the suspected offence because of her relationship with her son.”
The Criminal Cases Review Commission report which led to the murder conviction being quashed said: “The failure to record the incident on the custody record or otherwise reveal it to the Crown Prosecution Service precluded its disclosure (at the trial).”
It added that “... significantly, it would have exposed the favourable treatment Mr Chapman was receiving at the time that he was providing information to the police about Mr Maxwell and Mr Mansell”.