I sent the SARicon letter to The Co-operative Bank at the beginning of this month with regards to my old Smile account.

Today I received a letter from my new bank with my cheque enclosed; attached to the cheque is a note that states:

We are unable to locate the customers account details so therefore are unable to process this cheque.

Please return the enclosed cheque to our mutual customer.
My account number and sort code are clearly displayed at the top of my SARicon letter and they have written to me about my accounts in the past as well as using my details in order to offset funds to repay my card account that is in dispute so they're talking rubbish.

Also why have they returned the cheque to my new bank in order to return it to myself when the SAR letter states:

If there is specific information which you require in order to satisfy yourself as to my identity, please let me know by return. However, please note that the above address is the one which you normally use to communicate my private business to me and which you have hitherto found to be acceptable.
What can I do about this? How can banks blatantly ignore letters that are protected by law?