Well, I live and I learn!And before you resort to "the law is the law" that's not the case in this country - magistrates and judges have a degree of free will and interpretation open to them. "Ignorance is no excuse in the eyes of the law" is also an incorrect saying.
My understanding has always been that magistrates and judges do not have any degree of free will to interpret law, crafted over time and established through precedent and as such, cannot interpret the law in any way.
Where they do have discretion is in the application of any penalty available once guilt has been established with a few exceptions such as the mandatory life sentence for murder.
In magistrates' courts, the bench is usually comprised of lay people who may or may not have only the most rudimentary training in their role. The clerk to the bench is the trained person, frequently qualified as a barrister, has assistants and can guide the bench in all matters pertaining to case law and advise on sentencing guidelines.
As for the last sentence about ignorance, I think I'd prefer not to say to the beak, 'ang on a minute gov! I didn't know the law was like that so you've weighed me off too heavily!' It makes me wonder where these old adages come from.
I'm happy to be corrected in my understanding and there's at least one JP who has contributed to these pages. John Fleet is his name although I'm not sure if he still looks in. He could probably provide gospel on such issues.
Tom