Lack of service from Mix Telematics

I cancelled my policy with Mix Telematics because I found a better option for me. For the five years I used them, I never received any reports about my tracker unit. With the new company I get regular updated that the unit is working, I get reports of where my vehicle is, etc. I cancelled my policy on 19 May. I never even received an answer from them. I cancelled my debit order from end of June. And every time I received an invoice I let them know that the policy was cancelled. On 12 July a lady called me and told me that my account is overdue. I told her the story and she gave me another email address which I must use. I sent all the correspondence to them. And again today they sent me an invoice for an outstanding amount. I am not going to keep on paying for a service which I've cancelled in May already.