If you're going to convict someone of stealing £59,000, the very first thing you should have to show is that £59,000 actually got stolen. If there is reasonable doubt that the crime took place, no one can be guilty beyond that reasonable doubt. If the defendant claims the computer system got it wrong, it's not enough to say you are unaware of bugs, the prosecution should have to show that the computer's output was consistent with the results of doing the calculation by another method.