You should not waste the money unless you have something more than a suspicion.
1) The most likely source of error is the application of the wrong rate.
You know the margin.
The ECB rate history is here
So check to see have they charged you the right rate.
2) The next thing to check is what interest they charged you.
That is very easy. The balance @ rate /12 will be close enough.
3) Don't worry about the repayment calculations.
If they charged you the right rate and caculated the interest correctly, then you have not been overcharged.
In the very unlikely event that they have miscalculated the repayments, you will simply be paying off your mortgage a bit quicker or a bit slower.