Sold vehicle and the new owner has racked up fins with Horizon Parking

Half_way

Horizon dont issue fines.
If you know who the land owner is for the car parks in question and horizon are refusing to co operate, tell the landonwer in no uncertain terms to instruct their agents to stop this nonsense with immediate effect 

prowla

Do you have proof of when you sold the vehicle?

I would've thought that might include advert, conversation, bank deposit, acknowledgement from buyer, etc.

If so, you can provide that (redact bank details) and if the parking operator won't accept it, tell them you're happy for it to escalate to court, where it will be thrown out once you show it was not your car at the time.

(Save all of the relevant info, inc. screendumps for when you need it.)

As an aside, I bought a new car a couple of weeks ago and I logged the sale of my old one online, though admittedly my motivation was to get it in before the end of the month for the road tax refund!.