The Radio 4 podcast linked above is really good. The journalist who made it has added new episodes over the years as events have moved on. I listened to the whole thing a few weeks ago and was absolutely incensed. I knew about this before, but the more you know about it the worse it gets. Not sure what stage the public enquiry has got to, but looking at the coverage of that, it was a long line of former senior PO directors and managers saying 'I had no idea there was any problem' - really?
Was it Fujitsu operating the Horizon helpline, or the PO? Either way, something seems to have gone very badly wrong there, as those were the people telling the subpostmasters that nobody else was having any problems when quite obviously they were. I can't remember if it's in the podcast or an article I read but there was outright racism there as a former helpline staff member reported that some of her colleagues would answer the phone, mute the call and call out to the room (big open plan room) 'I've got another Patel on the line!' or words to that effect, and many of them would laugh and cheer. And then of course the call centre operative would fob the caller off, as we know. Where was the management? What kind of culture did they have there that this overt racism was not picked up and dealt with? Why didn't they notice or care that they weren't sorting out so many problems? Why didn't PO staff responsible for monitoring that contract take them to task over it?
(Racism also clear from the fact that subpostmasters from ethnic minority groups were much more likely to get custodial sentences, apparently.) 
Also, why on earth didn't senior PO staff and the judiciary notice that the number of prosecutions of subpostmasters for dishonesty had gone through the roof? As I understand it, the Royal Mail set up its own police force hundreds of years ago, before the country as a whole had any organised police, so that they could investigate thefts from the mail, which were and are common. The culture is that they are investigating theft, which is obviously a problem for a service handling parcels and letter post that may have valuable items or information. However, when also given the task of auditing subpostmasters they behaved as if they were still investigating a lightfingered postman or a gang stealing parcels. No thought given to the fact that subpostmasters were running their own small businesses and discrepancies could be down to mistakes or (as we now know, of course) computer problems. It's scandalous that in the 21st century they are able to prosecute without going anywhere near the police or Crown Prosecution Service.