It would be up to each individual charged to prove that he/she did nothing negligent or could have put in place a procedure that would have prevented the incident
I think, pedantically, that it would be up to the prosecutor to prove he/she was guilty, rather than the burden lying with the defendant to prove innocence.
All that has to be demonstrated is that there is "reasonable doubt" that they are guilty, not that they are innocent.
This may be small comfort as you don the cans, but it is an important principle of English law, and this thread is about the principles of English law.
W