That all changed with the Health and Safety at Work Act 2015. One of its key provisions is that it holds officers as well as corporations responsible, with strict liability, regardless of whether the corporation has plead guilty or not. So the actual people who set safety regimes, or undermine them, can be held responsible.
At least, that's the idea. Gibson's trial I guess will tell us whether the law means anything or not. And if it does mean something, I guess we'll learn whether National is actually still committed to that, or whether they want to return to the bad old days when employers could kill with abandon and walk free.