Here goes - lots of numbers to digest:
0 1 2 3 4 5 6 7 8
111000 107650 104133 100439 96561 92489 88214
83724 79010
5550 5383 5207 5022 4828 4624 4411 4186 3951
9 10 11 12 13 14 15 16
74061 68864 63407 57678 51662 45345 38712 317
47
3703 3443 3170 2884 2583 2267 1936 1587
17 18 19 20
24435 16757 8694 229
1222 838 435 11
0 1 2 3 4 5 6 7 8
111000 106450 101673 96656 91389 85858 80051
73954 67552
5550 5323 5084 4833 4569 4293 4003 3698 3378
9 10 11 12 13 14 15 16 17
60829 53771 46359 38577 30406 21826 12818 335
8 -6574
3041 2689 2318 1929 1520 1091 641 168
The first block of numbers is a repeat of what I ran for you last night, except that the annual interest payments are also shown. The annual repayment amount that I calculated, on which the balance reduces to zero after 20 years was £8900 per annum.
The second block of numbers follows the same format but using an annual repayment figure of £10100 per annum. You will not the rate at which the overpayment drives the outstanding balance down quicker, and the mortgage is paid off in under 17 years (the final figure at the end of the 17th year has gone negative, showing this has happened.
These are approx. right - based on the assumptions I used - an interest rate of 5%, and the model based on annual 'buckets' - some mortgages reduce the outstanding balance each month - not each 12 months, as I have done. But it should give you a reasonably accurate illustration of the impact.