Duncan Tomlin was 32 when he died after being arrested by officers in Haywards Heath in July 2014.
Following a lengthy inquest a jury has concluded that Mr Tomlin died from cardiac arrest following the use of a combination of drugs together with police prone restraint.
What happened to Duncan Tomlin?
During the inquest the jury heard how on the evening of July 26, 2014 Mr Tomlin's behaviour became irrational and erratic after taking a mixture of drugs and alcohol.
The loud disturbance at the house led neighbours to believe a domestic assault was taking place and they called 999.
When police arrived Mr Tomlin ran away but was chased and eventually restrained face down in the road, the inquest heard.
Mr Tomlin was 'screaming in pain'
Witness Jack Fulger told the inquest how he saw three to four police officers restrain Mr Tomlin on the floor.
He said: “He was screaming in pain. I’m not sure how you are supposed to restrain people so I wouldn’t be sure what to look for.
"But he was quite a thin guy, so he couldn’t have been hard to restrain. I thought it was a bit over the top.”
Jury finds neglect contributed to Mr Tomlin's death
Yesterday the jury concluded that neglect contributed to the death of Mr Tomlin.
They said: "There was an insufficient sense of urgency to move Duncan onto his side to address the risks of positional asphyxia from prone restraint couple with the use of handcuffs, limb restraints, the effects of Captor spray and the suspicion that Duncan had taken stimulant drugs.
"Duncan should have been moved onto his side earlier.
"Although the police receive training in positional asphyxia and the available policies extensively cover it, the efficacy of this training is inadequate.."
"The death was contributed to by neglect."
Family hail jury's verdict
Following the verdict, Paul Tomlin, father of Duncan said: "As a family we feel the finding of neglect by the jury is a damning assessment of the police’s behaviour.
"Coming into the inquest we had real concerns about information sharing, the need to move Duncan onto his side from the prone position, the use of handcuffs, limb restraints, and incapacitant spray, as well as the training that the officers had received in relation to positional asphyxia.
"Having heard the evidence the jury clearly shared these significant concerns. They have found not only that there were failings, but that there were gross failings.
"Throughout the past four and half years we feel Sussex Police and their officers have been arrogant, defensive and evasive. It has been incredibility traumatic to repeatedly have to watch the footage of Duncan in the back of the police van when we consider he clearly needed help. The jury have agreed.
"This is not the end of the process; the least we can do is continue to seek clarity and justice for our missing son and brother Duncan.”
'Damning indictment' of police actions
Helen Stone, a solicitor who represented the family said: "The jury's conclusion that Duncan Tomlin died due to neglect is a damning indictment of the police's actions in this case - and the way police treat vulnerable people generally.
"Neglect means causing someone's death by a gross failure to provide basic medical attention to someone who obviously needs it - but cannot look after themselves. In this case, the inquest jury clearly held that Sussex police neglected Duncan's urgent needs with the result that he died.
"All Britain's police forces have strict rules governing how their officers can restrain vulnerable people; and all officers should receive extensive training on how to abide by these rules.
"But the sad reality is when these rules are breached – as they often are - the only way in which those responsible can be held to account is by putting pressure on the authorities through the courts.
"Today's decision is a vindication of the Tomlin family's four year battle for accountability against denial and obfuscation from Sussex police and the bodies charged with overseeing police conduct.”
Police offer 'deepest sympathies' and vow to consider coroner's report
Assistant Chief Constable Nick May said: "I offer my deepest sympathies and heartfelt respect to the family of Mr Tomlin following his tragic death.
“All of our officers join the police service to protect the public and save life and it is of deep regret when anyone comes to harm.
“We accept the inquest's narrative verdict and will now thoroughly consider the coroner’s report and any recommendations within it.
"All of our operational staff are trained to recognise the signs of positional asphyxia in line with the national College of Policing syllabus. This training which, since Mr Tomlin’s death, is now offered centrally for consistency is reviewed and refreshed on an annual basis. "