You are within your rights to cancel a direct debit, or ask the bank to reverse one, if you are not properly notified of the amount and approximate date that the payment will be taken (it's part of the "Direct Debit Guarantee"). However, you are still liable for interest, purchases and other transactions although you can dispute any penalty charges that you feel are unreasonable.
It is worth establishing that the payments that have been taken are correct but I would not advise missing any more payments or cancelling the direct debit until the debt has been repaid (or transfered).
I would suggest that you try not to damage your credit record through this dispute until you have obtained another credit card from another bank and been able to transfer the balance to the new card. You can then avoid using the credit card issuer who has given poor customer service.