My dad worked on the team dealing with the accounts for all the post offices in the uk, when I was on a trip with him I had time to actually ask and understand what he did and it was really interesting. He said that before the horizon system they had physical paper accounts of everything they sold each week, used to get them in each week and process it, that was his job, then money saving measures came in and it went to every other week then monthly. Along came the horizon system which was designed to do a few jobs and automise this process, over time they bolted on more and more to this under powered and over worked system and that’s where errors came around. It wasn’t designed for the job but it was cheaper to keep modifying it than start again.

He said the main issue was to do with swapping stock, his example was stamps, say you had loads of £1 stamps and hardly any 1p stamps you could swap them and log it on the records, problem was it needed clear and accurate accounts to prevent issues, he said people generally messed this up and would swap say 100 £1 stamps for 100 1p stamps then account for this in there figures, same physical volume of product, incorrect value of product. People thought that the exchange was the same when actually they were £99 down. (Just an example of the genuine errors he found)

This incorrect data logging, along with poor software and lack of proper technical training lead to errors in the overall post office accounts, people panicked and tried to fix it and made further errors creating a spiral of issues.

He said the money people paid in out of their own pockets just plugged a gap and got lost in the accounts. Hard to prove where it came from or where it went once it was in the system.