What did Ed Davey and Keir Starmer know about the Horizon scandal?

The Conservatives and others are having a great time blaming Keir Starmer and Ed Davey for the Horizon scandal. This morning, on the Today programme, I heard that the CPS dealt with three cases when Starmer was in charge. These cases, sadly, resulted in prosecutions, based on the inaccurate information the courts were given. 

When a miscarriage of justice takes place on such a scale, it is right that we look into the causes. But just as we do not hold the judges and the juries in the cases to be liable for this, I do not think that we should hold Ed Davey or Keir Starmer to be liable either. People lied, many were lied to and too many lives have been ruined. Justice will involve trying to get to the truth, not blaming those with no blame.