Lack of response from the complaints department

I asked them to source a paid up letter for one of my accounts back in June 2019 and even escalated the matter to their complaints department or which they advised that they would provide me with a conclusive feedback on the 16th of July 2019. Almost a year later and I haven't received the feedback from the complaints department and now my clearance process is held up because they still haven't sourced the paid up letter.

I tried to call in and ask to speak to someone in the complaints department but the receptionist keep directing my call to customer service and they refuse to put me through to the complaints department.