Watched the four episodes yesterday and was appalled by the story - of which I was aware.
The flashing Horizon router lights were particularly sinister.

One thing I don't understand is why the postmasters didn't run a manual audit for 24 hours and compare it to Horizon's figures.

All they needed was an opening balance (ie £0.00), a note of incomings and outgoings (ie stamps - £1.00) and a closing balance. That could then have been checked against the actual cash for accuracy.
Assuming everything balanced, that would have immediately proved there was a problem with the computer program.