Steve,

I'm going to have a go at this for you to get you started. I'm useless
with interest rates so keep me on the right track as I go!
If the start balance is 2500 do you then add the monthly interest and
then subtract the payment made?

Eg. £2500 + interest of £37.50 - payment of £125 = new balance of
2412.50?

And do you want the results of this macro to produce on a new sheet?
You can e-mail if that's easier.