It’s worth listening to the BBC Sounds series on this.
They (The Post Office) did bring in forensic accountants (Second Sight) but as far as I can tell, they basically ignored them when the findings weren’t what the Post Office wanted. There are people interviewed on there talking about meetings ‘off the record’ as well.
It’s a long series and gets very, very detailed, so I’m struggling to remember it, but I also think they were basically worried that if Fujitsu were implicated then it would be bad for the big economic picture as they might pull out of other contracts, and it might even be bad for international relations.
Letting it get to this stage hasn’t helped much either though!