It was the third time Kennedy had faced a jury over the killing in Hammersmith police station in west London in 1990. In 1991 he was found guilty of murder but the Court of Appeal ordered a retrial last year and quashed the 1991 conviction.
A jury at the Old Bailey took nearly 12 hours unanimously to deliver its verdict yesterday. Mr Quinn, a labourer, had been drunk and taken to Hammersmith police station on Christmas Eve 1990.
Although the case has now gone through a first trial, a Court of Appeal hearing, an abandoned retrial last year, and a further 11-week trial, Kennedy's solicitor, Tim Cooke, said yesterday that an immediate appeal would be lodged. Mr Cooke said that 'lines of inquiry were being pursued that weren't able to be brought up at the trial'.
The appeal will again focus on claims by Kennedy's lawyers that they were unable to cross-examine a key witness whom they regarded as a 'suspect' in the killing of Mr Quinn.
Kennedy, a former restaurateur, shook his head slowly when the jury delivered its guilty verdict on the charge of manslaughter after finding him not guilty of murder.
Passing sentence, Mr Justice Swinton Thomas said that Kennedy, 47, had chosen to say that 'others were responsible'. Dominating Kennedy's defence had been his claim that a police officer had entered the 'tank' cell in Hammersmith and inflicted the horrific injuries on Mr Quinn.
In conducting his defence, Kennedy had chosen to try to blacken the characters 'of a number of quite innocent people', including police officers.
'They have been accused of being murderers, perjurers and conspirators to pervert the course of justice.
'Those people have been entirely vindicated. The jury's verdict means that they have found you and not any police officer or anyone else responsible for Mr Quinn's death.'
Mr Justice Swinton Thomas told Kennedy: 'You have shown no remorse for what you did - quite the reverse . . .
'I have no doubt that you must have known from the outset that it was you who were responsible for his (Quinn's) death'. Before the jury retired, the judge reminded it that killing without intent was not murder.
Kennedy's allegations that he was framed by the police, who then conducted a massive cover-up - which to succeed would have involved a large number of police officers on duty the night Mr Quinn was killed - meant the lengthy trial often saw a parade of officers in the witness box. Their backgrounds and personalities were dissected by Kennedy's counsel, Michael Mansfield QC.
The judge said in 'this very exceptional case' he felt he should say something about the way the police officers were attacked and their characters assaulted - 'and over whom a very serious shadow has been cast'.
Quite innocent people who had their characters 'blackened' had been 'totally vindicated' by the jury's decision. He mentioned in particular Constables Emlyn Welsh and Paul Giles, 'who had been accused of very grave misbehaviour'.
The judge added: 'They are ordinary human beings who may be fallible and may have made some mistakes but I have no doubt this sort of campaign must inevitably have had a very bad effect on them.' The effect on some of them, he said, was 'very plain to see'.
Yesterday Scotland Yard refused to make any comment.
Thames Valley police, on behalf of the Police Complaints Authority, conducted an investigation into the case in 1992. The Crown Prosecution Service is to decide if any officers will be prosecuted.
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