Overcharged on account

My latest MWeb account (through MTN) is R10 113.81. For many years I have paid R145 a month, for a 2GB cap, and hardly ever use more, mostly much less than this. The URL report shows that on 26th Sept, 11.5 GB was used. I have written to them saying that this must be a system error, as I was not streaming or downloading, or watching videos on this day, which I hardly ever do. My usual usage patterns are variable. On the 26th there are multiple repeated blocks like 312.2MB and 270.5MB. My usage shows up at 09:48, 14:44, 17:51. Multiple repeated blocks (270.5 & 312.2) are clearly a system fault. I have asked for this to be corrected, and they now referred me to log a query, but the query allows them only to explain my billing. I am not satisfied with the explanation they have given me, as I have been told that this is not human usage.