Office of Fair Trading
The Office of Fair Trading here
The OFT does not investigate your particular problem with the bank. They will only investigate the general principle involved.
The OFT seems generally to be unaccountable to the public. If you want to put pressure on the OFT you should probably write to your MP.
The OFT may be under a legal duty to investigate your complaint but they set no targets for investigating or producing a decision. They do not have to disclose to you the state of their investigation so far. However, this information may be available under the Freedom of Information Act. This has yet to be tested.
Dealing with the OFT tends not to be a very satisfactory experience
Calculating Fair Default Charges in Credit Card Contracts here
Calculating Fair Default Charges in Credit Card Contracts - Summary here