Acc closed on 19.08.15 still no closure letter rec

I settled my account on 19.08.2015 and requested that my account be closed. Despite a number of calls to the call centre requesting a settlement and closure letter, I still have not received it. I was advised last week when I called the call centre that the books close on the 11th of each month and that an urgent request has been sent to the closure department for a closure letter and that it will be sent to my updated email address by 11th September - Still no letter received - Why should a customer have to wait almost a month for a closure letter, when the account was not even in arrears -