"So what part of the Data Protection Act relates to someone accidentally viewing porn on someone else's computer?"

Simply a nine year lapse. Also your argument is facile. There was nothing accidental about scanning the alleged computer. If there was a case to answer it would have come to court many years ago. To hold evidence for nine years(assuming he can corroborate his allegations)is clearly a breach of the data protection act.
As has been stated elsewhere had a legitimate request been made about data held by the police concerning these allegations, the response would have been erroneous due to the actions of renegade police officers. If the act is to have any teeth at all it demands these officers be punished. There is not a lot of value in having a data protection act if anyone with a grudge can walk away and sit on data dubiously obtained in order to use it to settle a grudge many years later. Should the grubby, politically motivated, attempted character assassination by a disgraced, failed police officer be rewarded in any way? I think not.