After accepted cancellation of my insurance they still debit my account for 2 and a half years

I sold my house in 2016 and requested that my insurance on the house is canceled. I received an email where they accepted my cancellation. I received an account in the post that they are still taking money from my account monthly for the insurance of the house that I sold in 2016. I then requested a refund, they only paid me back for 2018. They are now saying that I had to send them the transfer document so that they could cancel the account (why I don't know). I want my full refund!