We have to acknowledge, for all our nationalistic pride in things British, our legal system isn't designed to arrive at the truth; it's an adversarial system where one side has to beat the other side, convincing juries and magistrates and judges for that matter of the veracity of their claims.
CPS employs an expert witness in the form of a forensic testing lab not to find out the truth but to win the case they are paying to prosecute. Why should they continue to patronise a firm that persistently doesn't deliver the goods.
Of course they have a vested interest. They are employed by one side.
Now I'm no lover of all things French but at least you have to hand it to them, their magistrates "investigate"; in other words, they seek to find the truth.