Cancelled Policy

I have been trying to cancel my life cover with 1Life for months now. I have emailed a signed cancellation letter more than once, also called the call centre however they continue to debit my account. I demand all my premiums be refunded immediately. The reason I cancelled is because after paying for so many years they suddenly emailed to say my cover terms have changed because I did not go for blood tests but I don't recall ever receiving that type of request. They email all the time or will phone with haste to sell the products so why is it so difficult to cancel???