CANCELLATION NOT PROSSED AND WANT CLIENT TO PAY FOR THEIR ERROR

I had a contract with Mix Telematics which had expired and I was on a month to month contact. on the 16/08/2022 I sent am email for Cancellation and I received a system auto reply confirming that they have received my email. in August and September which is perfect. In October my account was debited again and I reversed it as they had no business going into my account while the contract has been cancelled. I have been receiving calls from them I explained what happened and I also give them the case number, they keep on saying that they will send the email to escalate nothing happens, now I have received a final demand for something I have cancelled and served a notice for.