Easy fix to this one -

(1) Keep all documents INCLUDING your diary records that you have made of EVERY phone call you have made to them  Diary records can become a legal document and WILL be recognised by a court.

(2) Let them bill your credit card for any amount they like.

(3) The very second they bill you do two things:

(A) File a complaint with the Telecomunicions Onbudsman (easy to do online) and
(B) Contact your credit card company and demand they reverse the transaction as it was not authorised by you.

Taking those actions will get the wankers talking to you I can assure you from first hand experience (not this company but a very similar situation with a shonky Telco)

As you have found you are wasting your time talking to them......