Customer service nonexistent

On the 24th of November I had signed an agreement to trade in my Hyundai I. 10 for an Isuzu swift. A ttade meant i would no longer have the vehicle (Hyundai I. 10), in my possession. As such i contacted my insurer advising them of the changes immediately. The lady seemed efficient enough. She mentioned she would have my last day of cover on the Hyundai I. 10 as the 29th which was the following Monday. The Isuzu was to be collected on the 26th. I even requested a confirmation of cover letter twice as i was not getting it despite being promised it would be sent. A red flag of incompetence that i should have never ignored. Nonetheless time passes i didnt receive an policy updated policy schedule advising that the Hyundai was removed. My policy schedule still indicated that i had two vehicles in my possession instead of the one. Concerned id be debited again in December for full premiums for insurance i no longer have. I sent an email to my insurer telling them that i do not have the Hyundai I 10 therefore i should not be debited for it. My debit runs on the 20th of the month. In November a full premium was collected on the 20th despite not having the car the whole time but i was determined to not be wronged in December. So i emailed them on 12th December. But there was no response from the insurer. Sure enough on the 20th of December i was debited for both vehicles. I then sent an email again to their customer service and complaints mailbox which seemed to get their attention but not enough to get back to me with a resolution let alone a policy schedule confirming the amendments. If i could, i would have reversed the debit order already and went about my life but im not able to do so. But one thing is for certain their customer service is not upto scratch and i should start looking into getting a new insurer as they have failed to provide basic customer services, what then will happen should i have to claim?