@Jollyman
I used the uk site
Mortgage Overpayment Calculator for my calculations. The numbers are in £ but its the same calculation
331308 for 23 years at 3% puts the repayment amount at 1660 per month and 458,980 in total
Paying 35k off this today, and keeping the monthly repayment at 1660, the total amount to be repaid is 427,770, and you clear our mortgage 3 years 3 months earlier.
Thats an overall saving of 31,210 over the lifetime of the mortgage. However that does not take into account the time value of money
I cannot see a 4k comment - but I do see a 1200 saving in interest per year => 35% @ 4% - if that helps