Thank you, that what I needed to know. I'm not averse to challenging tickets and fines - I've successfully done several already, but only with the information to know whether it was worth fighting. What I didn't know here was whether there were grounds to do so. If it's not a viable option, I'd rather not spend the time on it. If, however, it is viable, I'm very happy to try. I've posted here to find that out.