Not against the software developer? Let's wait and see whether they did due dilligence, shall we?
This happened over a very very long period of time.
People committed suicide over these false charges. Ruling out prosecuting the developer if it can be proved that they knew, did nothing, colluded in silence, etc., would be insane.
FWIW, I don't think "public prosecutors" in the conventional sense (Crown, CPS etc.) were even involved here. The Royal Mail is (like until recently the RSPCA) one of the few entities in the UK entitled to bring its own prosecutions. The Royal Mail pretty much invented the concept of formal investigation and prosecution. This is the Post Office acting with the assumed prosecutorial power of the state; the CPS may not ever have been involved.