However in most of the cases I have read about, there is evidence that has been deliberately withheld from the defence and from the court. Maybe these sorts of results (where possibly guilty people are released) might make the police and prosecution think twice before using this sort of tactic.

Does anyone know if there is a time limit (and what it is) at which a retrial can no longer be ordered after appeal as opposed to declaring the conviction unsafe and releasing the person? Or is it down to the discretion of the judge and the individual factors in the case?

(sorry for the thread drift)