The problem with the police case against Cannan is that it has always pointed to suspicions of his guilt that are either outlandish ("I remember seeing Cannan looking in Sturgis' window 14 years ago"); knowingly untruthful (insinuations that he was the Birmingham house-for-sale rapist - who was, however, described by victims and nothing like him); or entirely unpersuasive ("The police are convinced Cannan had access to a BMW" - they were also convinced Jean Charles de Menezes was Asian and carrying a bomb).
If the case includes stuff that collapses in a nanosecond when studied, it makes a rational person sceptical of all of it, even if better-founded.