Hi Dr
Step 1 is for the bank to apply the correct interest rate on your loan and to recalculate what the balance today should be. They should then issue you with a new set of statements from the time of the interest error. So you will see your repayments and the interest charged each quarter.
You can now check this to see if it is correct. You can calculate the quarterly interest fairly easily if you are happy with a rough approximation.
If you are happy with this, that you owe the lower balance, then you don't absolutely need to do anything further.
However, if they had applied the correct interest rates all the time, then your repayments would have been lower. You have made higher repayments, so balance today is lower than it would otherwise have been.
So, they should go a step further and recalculate what monthly repayments you should have made, had they used the correct interest rate and what the balance would be today with those repayments.
You will now have two figures
Corrected balance using correct interest rates and actual repayments: €105,000
Corrected balance using correct interest rates and correct repayments: € 110,000
You should be offered a choice of
1) Your balance reduced to €105,000 and no further action.
2) Your balance reduced to €110,000 and a refund of €5,000.
You might have a case to argue that ran up credit card debt at 15% because you were overpaying your mortgage. I don't think that the bank would accept this so you might have to go to the Ombudsman.
Brendan