You can write to say that, as you cancelled within the appropriate cooling off period, you are entitled to a full refund under the distance selling regulations (avoid using phrases like you "believe you should be"). The letter can mention dates and content of phone calls, the date the policy was taken out, payment details, details of the misleading aspects of the policy, etc..
If they write back to say that they are entitled to the charge or the distance selling regulations do not apply or other reasons why you don't get the full refund, you can either accept what they say or you can contact your bank to dispute the transaction through their process. As part of this process, the bank may wish to see copies of correspondence. If the bank agrees with you, they can reverse the payment (or, at least, the difference between the payment and the refund).
If they don't write back within a month, you can reasonably go straight to the bank.