Thanks all for your efforts in helping me resolve this, glad I didn't ignore the initial email as phishing. Just in case this happens to others, I received the following explanation:
"Sorry I didn’t make this clearer; I will try to explain from the info I have. Firstly I should explain that there was no body-worn video recorded by the inspector that issued the notice. I had to use footage recorded by another inspector on the same carriage who was dealing with another passenger. This was enough for me to ascertain that the passenger was not you, but I could not make out much of what was discussed between the inspector and your impostor. In most cases, it is someone who knows you and would be familiar with your details. As your details were already on the system, it is hard to tell what was supplied by the passenger and what was assumed by the inspector, but your name will have been supplied by the passenger. I am quite confident that the passenger knowingly provided your details, as opposed to your data being breached from anywhere. It is also possible that the passenger is named '[my name]', and the inspector believed that your existing account was theirs. As I say, it is hard to tell from the limited BWV available. However, your name will not be accepted again by our inspectors. You can report the matter to the Police via this link if you wish
https://www.actionfraud.police.uk/"