I think you sums are irrelevant (you suggested it).
If you had a mortgage of �100k and the interest rate is 4%, you have to pay �4k pa in just interest alone. But if you overpaid by �2k in the year (making a total payment to the lender of �6k in the year), the extra comes of the capital sum. So at the end of the 1st year, the outstanding balance is �98k.
So in the second year, the interest payment alone is 4% of �98k, which is �3920. If you again paid the company �6k, this time �2080 would come off the capital sum (6000-3920), so the outsyanding balance is now only �95920.
In the third year, the interest payment would be 4% of �95920, or �3837. So you again paid �6k, the capital has now reduced to �93757.
The common thread here is that every year, the amount of decrease in the capital sum escalates - �2k in the first year, �2080 in the 2nd year, �2163 in the 3rd year.
I haven't a clue what you are trying to prove with the 6% figures. Just remember that the more you can afford to pay off the capital sum, the quicker it decreases.