OneDrive has a feedback option.
I eventually found Office 365 live chat who directed me to the right page, site search completely failed. I suppose loading the app would have been easier but I have no idea if I even have it installed. It was more I was panicking that it had somehow been backing things up I didn't ask it to as I don't actually use it.
Response to my feedback was "its fine, just ignore it" which is totally not acceptable. They need to fix that e-mail to say "you MAY be over your new storage limit" rather than saying "you will be over your storage limit".