One way of looking at lump sum payments is that if you don't make it you will be paying interest on that amount for the entire duration of the mortgage, i.e making a lump sum payment of €30,000 now means that that €30,000 would have been the last €30,000 that you'd pay off (in twenty years time or whatever) and you be paying interest on the €30,000 for the twenty years.
Interest at 4% per year on €30,000 for twenty years is at least 80% of the €30,000... = €24,000... that's the minimum you'd save. (This doesn't consider compound interest which means the actual savings are greater)
Cheers
Joe