On Jun 26, 8:33 pm, I wrote:
Again, this presumes that 6% is a nominal interest rate. I wonder if
in the UK, interest rates are specified as "APRs" -- that is,
compounded ("effective") interest rates. If that is the case, replace
1+6%/365 with (1+6%)^(1/365), which can be computed once in an
auxilliary cell.

Wrong! I don't know what I was smoking at the time, but that has
nothing to do with mortgage APRs.