Is it registered to the Hire company? Have you checked with them?

No, it wasn't registered with the hire co. It was they who told me someone might have put it on their account and forgotten to take it off.

Two thoughts here.

The first is that if van is indeed on somebody else's account, and you know it's not the hire company, you'll probably hear nothing more. Even if charge appeared in their current account plenty of people wouln't notice for days/weeks. As DartCharge uses a prefunded account there's another buffer of inertia in the mix. I rarely check mine as we only use the crossing a handful of times a year.

When the find out they've been charged by mistake DartCharge will tell them it's tough luck. Vehicle was on their account.

DartCharge were right to refuse to discuss registration. Mix of basic data protection and need for issues to be resolved in writing.

My second thought may be less welcome.

This is an example of how members of the public get themselves in unnecessary traffic penalty issues. The vehicle was already registered and not eligible for discount on OP's account. Rather than just pay the charge conventionally, and the discount must be small beer compared with van hire, he chances a fine. If he gets one he'll struggle to win an appeal.

If a fine arrives he MUST either pay up OR appeal.

A lot of people hope they can argue their way out later or it will go away if they ignore it. While private land operators sometimes give up public bodies never do. They always go to Traffic Enforcement then bailiffs. By then the debt is in hundreds and cars at risk of being siezed.

At that point they finally seek advice. In such cases the invariable advice is agree installments with the bailiff. It's way too late for anything else.