Bang in a complaint, "horrified to read", "besmirch my good name", "have always prioritised paying UB", "after all my struggles to meet my obligations", "never been in arrears",
Actually you don't need to go all Talk to Joe about it, you can just state that this is a serious error and you want confirmation that it has been rectified and compensation for the distress. Damage to your reputation is a serious matter, although the letter was presumably private, who knows who might have seen it in the bank.
You really should complain, UB have lived with hopeless IT systems for a long time now. Only pressure will make them invest in updating them.