"Michael Marshall" wrote:
joeu2004 Wrote:
For the terms above, I used Goal Seek to determine that the initial
payment is about 4348.97, and the last payment is about 6200.59.

Note: I assume that the investment growth rate of 4% is an annual
yield when compounded monthly (the payment frequency). In other
words, the monthly growth rate is (1+4%)^(1/12)-1.
[....]
Are you sure you didn't make a typo because I am getting an initial amount
of £4328.97

No typo. The difference is explained in the footnote above.

Since I assume the investment growth rate is an annual yield (compounded
rate), the monthly growth rate is (1+4%)^(1/12)-1.

Apparently, "you" (tadPMT from tadxl.com) use a monthly growth rate of
4%/12.

The latter is incorrect because it results in an annual yield of about
4.0742% = (1+4%/12)^12-1.