Incompetent Santander - what to do?

ejr2403

A year ago I opened a personal account. never used it, thought I'd closed it until I saw a default on my credit file
Phoned Santander. They had registered a default against me because of non-payment of a monthly fee for a current account. Agreed with them that it was partly their error. They were sending statements and overdue letter to the wrong address even though I do also bank with them commercially. They gave me a payment of £25 as a hardship payment and promised to remove the default or at leas mark it as satisfied and advise me accordingly.
To date after 15 phone calls 12 emails etc the default remains, I cannot get a mortgage because of it, I cannot satisfy it a it is already satisfied and to date I have not had any form of reply or explanation from Santander. Emil have been promised but never arrive and the mail has never brought any of the 5 promised letters to my door...I hate Santander! I've lost my mortgage rate!

What on earth do I do now?