Just had EXACTLY the same today. Final notice - had to pay £101 (no previous correspondence) - We went through on the 18th Jan.
We pulled over to download the app, register our car, add funds and pay - we thought we had.
However I called the number on their site and spoke to someone, who could see that the above was correct but the car didn't seem to be registered so the fee didn't go out. She amended it, closed the fine off, and took the original £1.71 or however much it was.
Definitely do call them.