Chasing a debt for a parking ticket, 2 years after...

Chasing a debt for a parking ticket, 2 years after we sold it. Provided 2 proofs (sale receipt/DVLA Letter). But apparently not enough. They asked for a letter from insurance company too. There was an issue with this letter, but instead of responding to me as they could see this was a mistake, they instructed bailiffs. Horrible company run by horrible people.