If we are talking about offences as old as 18 years then at least some will pre-date the current legislation, which is a whole other can of worms.
However, in relation to the current legislation, whilst I remain of the view they cannot simply claim the fines as a debt, it seems that the legislation is drafted such that if you simply ignore them then they are obliged to suspend your license, so you do need to dispute liability for these offenses, which would be a reasonable response if you were unaware of them. If you take that step, (assuming you are within time to respond to the notices or they grant you an extention) then in my view they can do no more because once you dispute responsibility, if they wish to fine you or suspend your license they need to initiate legal proceedings, and then section 31 applies to prevent such prosecution.
As to section 27, is simply a section that requires the authority to issue a reminder notice after 28 days if the subject has not responded or the notice been withdrawn. It has no effect on the application of section 31 since it only pertains to reminder notices. Failure to respond to the reminder notice can, however result in the suspension of your license pursusant to
Unfortunately the legislation is somewhat circular in its drafting and whilst it specifies a limitation on when proceedings can be brought for such offenses, it does not clearly establish time limits for suspension or other enforcement action for non payment, however if you can dispute the offenses referred to in the reminder notices, they it would appear they may be unable to take further action against you.
If they do take further action or if they refuse to accept you are entitled to dispute the notices, then I would suggest you engage a lawyer to fully review your case. The law is somewhat complicated, so a thorough review is warranted, but I strongly suspect you have a reasonably arguable case, and the irregular circumstances of old infringements being raised so many years later suggests to me that you would get the sympathy of the court.
I appreciate this may not be the clear cut answer you were hoping for, but it does reflect the complexity of the law, expecially when dealing with unusual situations and I trust at least assists your understanding as to how to proceed.
Good luck and please rate my answer.