Hi Masterboy
There was some confusion over your question and I have deleted the replies trying to figure out what you were asking.
I think you are asking if you lose the fixed rate when you make a partial capital repayment? (If that is not what you are asking, please clarify exactly what your question is.)
When you make a lump-sum payment against a fixed rate mortgage, the remainder of the mortgage is not affected.
So taking your example.
Let's say you are on a fixed rate of 3% with 5 years remaining.
Let's say that the early repayment fee for the full mortgage of €100k is €4,000.
If you make a capital repayment of €20,000, you will be charged an early repayment fee of €800.
The remaining €80,000 will continue at 3% for the next 5 years.
Brendan