Your story of an attempt to charge you for someone else's error reminds me of the way that the Woolwich Building Society treated me when (by handing them a banker's draft for about £35k) I thought that I'd paid off the mortgage on my house. Several months went by before they wrote to me, stating that they were about to repossess my house because I'd not been making any payments on my mortgage!
When I went into their office, the branch manager was very apologetic and eventually managed to find the missing money in their accounts and credit it towards my mortgage. She then told me that I owed them over £200 because of the interest that had built up since I'd given them my cheque!
Like you, I kicked up a fuss but it was only when I pointed out that the offices of the local newspaper were just a 2 minute walk from her office (and that I was sure that their journalists would love to publish my story) that she suddenly found a way of cancelling the interest.
So I received confirmation that the mortgage was paid off, together with the deeds to the house. Over a decade later I tried to sell a small piece of my garden to a neighbour, only to be told that I couldn't do so as there was still a charge on the property, registered to the ruddy Woolwich Building Society (which, by then, didn't even exist - trying to find someone at Barclays Bank, who took over the Woolwich, to sort out the problem was a nightmare!)