If I understand you correctly, you have overpaid the mortgage, but this money is sitting in an account and has not be used by the bank to reduce the capital owed on the mortgage.
If so, you are paying interest on the mortgage and getting interest on the credit amount, which are really just savings in an account.
You need to find out what interest these savings are getting. They may only be getting a current accountrate of <1%.
If the interest you are getting on the savings, is less that the interest that you are paying on the mortgage then you are losing out.
You have two choices
1) Ask the bank in writing to use the savings to reduce the capital.
Pros - your mortgage capital is less, so the amount of interest you will pay over the rest of the mortgage will be less. In the event of the ECB rate going up, this saving could be significant
Cons - it is difficult to get access to this money if you ever fall on hard times.
2) Leave the situation as it is.
Pros - if interest rates go up, you will not have to pay a larger monthly mortgage payment, as you can dip into the savings to pay the difference.
- If you lose you job, or have a pay cut, you can use this money to pay the mortage.
Cons - the money may not be getting much interest the meantime, so will lose value against inflation.
You might be happy to do option 1, if you have a rainy-day fund built up or you might be more
comfortable with option 2, or a combination of both.