If all else fails do you know a friendly lawyer who could pen a letter on your behalf (or maybe an unfriendly one! 🙂)? ..... surely the potential consequential losses and distress and worry to your elderly parents along with your time spent dealing with things billed at your highest rate could be leveraged to a decent amount? ..might focus their minds on actually doing something about it! .... ... the mere fact they don't have a transparent and prominently published procedure for resolving mistakes in their database must leave them legally wrong-footed and vulnerable?

K