The Dart Charge is nothing to do with TfL. I've just had 2 Penalty Charge Notices from Dart Charge, the letter has a big red circle in the top left corner with a white C in it, the top right says Department for Transport with a government logo. Large letters underneath says PENALTY Charge Notice (PCN), do not ignore.
My matter was resolved amicably with a phone call, I couldn't register my new car with my Dart Charge account as it was already on their database with the previous owner.