Overcharging for 2 years

I recently found out that Standard Bank has been charging me credit card service fees that was already included in my Prestige Account package. After checking each of my statements, I found that they were doing this for 2 years since November 2015. I queried this and they mentioned they would definately refund me the overcharged amounts. However, when the refund came through, it was only for the last 6 months. I contacted them again and they said that they only process refunds for 6 months. Not even an apology. I've been with Standard Bank for over a decade with many accounts and this is how they treat loyal customers. A brilliant strategy. ***** from your customers and if you get caught out, have a rule that you only pay back 25% of what you stole. I don't mind that they made a mistake, these things happen, what really irritates me is their refusal to rectify the situation. Please can you refund me the full amount. This is not the way to treat longtime clients.