Unauthorised debit order

I cancelled the policy in December 2018 but still they took money of my account in Jan and Feb 2019 . Then it stopped and started started again in Sept 2019 . I have called them to refund but they refuse saying they have no record of the cancellation in Dec 18 . Numerous emails and telephone calls and still no refund . I have reported it to the bank . Their reference number on the debit order and value changes every month . So to put a stop to the debit order is difficult as they never use the same value or reference .