The family of a man with mental health issues who died after a belt was placed across his face in police custody have expressed dismay after a judge said he could not be sure that the method of restraint contributed to the death.

Thomas Orchard, 32, a church caretaker, had a cardiac arrest and died in hospital seven days after being arrested and taken to a police station in Exeter in October 2012.

In a landmark prosecution last year, the office of the chief constable of Devon and Cornwall police admitted breaches under the Health and Safety at Work Act but disputed that the belt contributed to Orchard’s death.

After a three-day trial of issue at Bristol crown court, the judge Julian Lambert concluded on Thursday that he could not be certain the emergency response belt (ERB) contributed to Orchard’s death. He will sentence the police next month for the health and safety conviction.

Alison Orchard, Thomas’s mother, expressed concern at the ruling. She said: “We have spent the past six and a half years watching the same CCTV footage of Thomas dying, listening to witness statements, reading reports, perusing documents which relate to the safety and wellbeing of those in police custody and witnessing the defensiveness of those representing Devon and Cornwall police.

“We have consistently told those in authority how we perceive what happened to our much loved son and brother. We have always thought Thomas’s death was a needless one, caused directly by sloppy and dangerous practices, most especially the application of the restraint belt around his face and head, as well as the lack of care and negligence from the very organisation which should have been protecting him.

“As a family we are nothing but dismayed by the judge’s decision. It is hard to believe after all that we have witnessed. More than anything, we have always been hopeful that Thomas’s death would not have been in vain and that lessons could be learned from it.”

During his detention, Orchard, who had paranoid schizophrenia, was restrained and an ERB was placed across his face. The restraints were removed and he was left in a locked cell, where he lay apparently motionless for 12 minutes before custody staff re-entered and started resuscitation. A postmortem examination later found he died from a brain injury seven days after the cardiac arrest.

In his ruling, Lambert said the burden of proof lay with the prosecution, who had to show the belt played a “significant” role in Orchard’s death. He said it was not clear from the CCTV evidence that the ERB obstructed Orchard’s breathing.

Victoria McNally, a senior caseworker at the charity Inquest, which is supporting Orchard’s family, said: “For six and a half years Thomas’s family have been fighting for some level of accountability, a delay which can only point to the failing systems of justice following deaths in police custody. Today’s decision is a bitter blow with no one held criminally responsible for causing his brutal and preventable death.”

Inquest said it was the first ever guilty plea on health and safety charges from a police force in relation to a death in custody.

The family’s solicitor, Helen Stone, of Hickman and Rose, said: “We should not lose sight of the fact that the Devon and Cornwall force has admitted breaching health and safety law, a crime for which it will now be punished.

“It should lead to all the country’s forces reviewing how equipment is approved, reviewed and trained for use to ensure not only that they comply with the law but that no other members of the public are put at risk as Thomas was.”

Shaun Sawyer, the chief constable of Devon and Cornwall police, said: “This matter is not complete as we await sentencing on 3 May. It would be inappropriate to comment further until that point. My primary thought is for the family of Thomas Orchard. The last six and a half years is something I and no family would want to go through.”