Tax return online. Is there a calculation error? - NOT amateur radio

I have just completed my UK tax return online, but I think there is an error with the calculation algorithm when working out interest on bank and building society accounts. I need to double check but I am pretty sure I am being over-charged. If I am right then millions may be owed to people. As I said, I need to check, but I shall certainly write if I am correct. Also, yet again, I got caught by the stupid part asking if you have any other comments to make. It said I had 1 error but would not tell me where the error was. Whoever designed the form needs the sack or re-educating. Surely in this day and age we can expect a free-form form that you can just write in? Surely this is not too much to ask?