Fujitsu's responsibility was to deliver code that met the requirements, on-budget. It was the Post Office's responsibility to ensure that the software was fit for purpose. And in fact they knew it wasn't; but they accepted it anyway, probably because having it re-worked was going to push them over-budget.

The PO's requirements for the counter-mech project consisted of three yards of requirement documents, in ring-binders. I spent several months in a room with no windows, working through those requirements. They were mainly forms, supposedly describing inputs, outputs, reports, and screens. MOST OF THEM WERE BLANK.

I wasn't with Fujitsu; this was an earlier iteration of the project. But I have no reason to believe that a step-change occurred in the way the PO handles IT projects, between when I worked on it and when Fujitsu picked it up.