There was one post master who claimed to have been charged upward of 30 pounds in theft. There were others with large sums as well that went missing. If they believe there was a bug in the software and got together I'm sure they could find the bug, clear their name and seek restitution for what they had to pay back. If enough of them got together and bought even a couple guys I'm sure they would be better off financially as well as having their names cleared, keeping their houses and contracts.
That's options 1.
Option 2 is to have someone start a business that analyses specific problems with the postmasters. They can look at the code and even go over policies and procedures with the post masters. Charge a licensing fee for the extra support that you offer, have it be an unbiased third party.
With the amount of money at risk to being lost and the number of sub-post masters that they were talking about in the article it wouldn't even have to be a large amount of money from each sub-post master to sustain someone specifically to look at the code for them.
You talk about communism but I don't think the moment you start working together it's communism nor is it a bad thing. This isn't 1950's U.S. this is 2013 UK.
I'm not saying they should be open source or not, they could give access to the code to a third party company so that they could trouble shoot it anyways with NDA and all that jazz. In the end the story here is that these folks are apparently being screwed by the post office and they have no recourse to prove it. The same thing could happen to anyone using closed sourced software to do accounting or other sensitive work. One bug could land people in jail and your are shit out of luck to prove it was a bug. Not having the option to hire someone to look at the code is scary in that aspect.