�12 seems to be the accepted standard now - many institutions have lowered their charges to �12.
You've had more than a computer generated letter - you have had money that isn't yours and you are not entitled to, breaking the terms and conditions of your contract.
It is easy to avoid paying these charges - set up direct debits to pay the minimum each month so you can't miss a payment, and keep an eye on your balance so you don't go over your limit
You could try claiming them back.