I'm definitely not a senior member, but will do my best to reply.
Firstly, you must mention it.
I have the feeling that you do not know what you signed. So just describe the events. But I would be careful claiming that you forgot to pay. What you signed may give a different version of events.
Would an FOI (Freedom of Information request) help if you have no documentation related to the incident in question? I'm not expert enough on this subject to give an opinion here. But you can still send an email immediately describing what happened, then follow this up later if you find out exactly what you signed.
Make sure your English is checked before sending the email.
You can email
furthernationalityenquiries@homeoffice.gov.uk with Name, DOB, Nationality, Reference. Keep the email as proof. You will be fine if this is the only blot on your record.