The 'server' piece seems to be based around an XML message store (hey it was the 90s dont forget, XML was the big new thing) called Riposte build by Escher Group.
There were some touchscreen EPOSS terminals for the post offices - these may also have been part of the Riposte system. Then there was a bunch of other stuff built around that and written in VB6 (which for the time was a reasonable choice unless you wanted to go Java or C++) running on NT workstations.
Syncronisations from the thousands of post offices to the central server seemed to be via scheduled programs that picked up local message files (probably xml) and sent them to the central server over ISDN lines.
Centrally there was something called the 'cash account' which pulled in those messages and (I guess?) added them to the riposte xml message database. As mentioned in the witness statements, they had no agreed data dictionary or schema for all the XML stuff so no centrally agreed scheme of how to represent all the different messages. So somewhere something was probably written to 'transalte' all these different messages into some nominally agreed standard.