The key clue

 This is a piece of evidence, preferably something that is indisputable because it is scientific, which is an anomaly in the case. You need the key clue, because the moment that you start to approach witnesses - or experts - who might help you, they are going to be sceptical - and in some cases give you a long lecture on why you should not be interfering with the "true" course of justice

Your tactic on such occasions is to sit back, let them have their say and then ask them to explain the "key clue".  It is only when people realise that not everything that went on in the trial is the whole truth that they begin to cooperate with any re-investigation.

The key clue should be based in science, not in witness evidence. Finding  a witness with a story different to those who appeared at the trial is simply not enough. When you are dealing with witnesses, you are not dealing with the kind of rational people you will later encounter in the Criminal Case Review Commission (CCRC). Witnesses are usually emotional - and their emotions are generally biased against you. Science is not something that they are accustomed to considering in the account of a crime - they look for motive and witnesses, something to believe in rather than something that stands up to scientific testing. That is, after all what they generally see in detective dramas  on the TV.  So choosing a key clue based in science is not simply because witness evidence is generally less successful at the CCRC and in the Court of Appeal., but because  it is also a means of gaining the confidence of witnesses.