Pathetic customer service! Can't get a settlement letter out of them

Absolutely shocking service. I've settled a credit card early in November and accidentally overpaid the account but I have been struggling to get a simple settlement letter from this pathetic company so my bank can refund me since 5 Nov!! Every time I phone I get the same reply that it's been escalated - BUT in the same breath they say it normally takes 7 - 14 days to get such a letter - but this is more than a month later and still no letter! They are very quick to phone and harass you if you are late of missed one payment, but no joy in getting any response for a simple letter when the debt is settled!