Condition (b) hardly ever applies. However, I'll try to think of an example which might apply for this type of offence:
The defendant might plead not guilty on the grounds that he wasn't in a public place. The prosecution might argue that they were sure his location met the definition of a public place. At this point, the magistrates would probably seek advice from the clerk of the court regarding the relevant legal definition. If the clerk could only find test cases where the situation was similar but not identical, the magistrates might decide that it required a judge to determine the validity of the law for the particular circumstances referred to in the charge before them. In which case, they'd rule that the matter was beyond their powers and refer the case to the Crown court. (Note that the 'powers', referred to there, relate to the magistrates powers to make decisions about the law. That's completely separate from the situation where they refer a case to the Crown court because of insufficient sentencing powers).
As I've indicated, such a referral to the Crown court would be extremely rare. (Normally, magistrates either know the finer points of the law or they can get the relevant information from their clerk).
Chris