You need to use another hosting site; the one you used no longer covers the UK
So how incorrect was the entered reg number ? You can argue on 'O' and '0' (zero), because the official font for number plates has no zero character, the letter O is used. If it was for a totally different registration, then it is mitigation, I'm afraid, despite full payment being made. London councils and TfL between them are collecting well over £600 million a year from PCN penalties. So they're not going to stop the money rolling in.