Six months ago, BMS (Barclays Merchant Services) changed over to a new accounting system. So when the first new bill arrived, in a totally different format from the old bill, I compared the old with the new, and I found a major discrepency. Previously, they had been charging me £50-£60 VAT each month. Suddenly, the VAT number was £5.
So I called them up. Either the old figure was inflated, or the new figure was too low. It took them about six months to look into it, and eventually, I got a nice refund from them.
I wondered, then, what they would do about all their other customers. Now I know. I just got a form letter from them, explaining about this.
This blunder must have cost them a lot.