The other complication was that a change in the law was made to assume computer systems were correct by default due to the problems with the existing law around breathalysers and speed cameras:

"In 1997 the Law Commission published a paper which went into some detail about the use of mechanical and computer evidence in court. It seemed a little too fixated with the effective workings of speedometers, traffic lights and breathalysing devices called ‘Intoximeters.’ It concluded that the present law is ‘unsatisfactory’ because of the necessity for prosecutors to ‘prove that the computer is reliable.’"[0]

The amended law changed the burden of proof from the prosecution proving the system functioned correctly to the defence proving it didn't, without access to the systems being used to prosecute them.

[0]: The Great Post Office Scandal, Nick Wallis