As advised above, you will need to get all your documents and facts lined up. Magistrates are not legally trained, they rely on the advice of one court clerk, which is actually a qualified barrister. If this clerk is a keen driver, they will understand and give good advice. If however, they do not like driving, get cut up by motorists on their bicycle every day, etc., they may interpret the law differently. As you work for ATS, you may be seen as supposed to be the expert here, advising your customers. Have ATS a legal department that can advise, as they would have to do so if you, or a colleague had advised a customer who was now in this situation.
You need to stand in court, confident that you are right, with all the details to back it up, without making anyone else look stupid. (Policeman, clerk, magistrate, etc.)
Might be an idea, between now and court case, if possible, to refit original wheels and tyres, showing that you are concerned to get it right.
Then, after you've won (or sadly lost) fit some arch extensions.