I got the guy to explain the formula they use, and I turned it into an equation. I then solved the equation for £50 and got a number. Gave that back to the accountant, he uses that as the starting figure and out pops £50.23.
Worked through the working of the accountant, and there it was: this rule + that rule created a step function. Arghhhhhhh!!!!!!!!!

So, I then started on how much I wanted to pay in total each month. Worked that through the formula, out popped an answer. Plugged it back into the accountant, and blow me, but I could increase my payment each month by exactly £50!
Taught me an invaluable lesson about accountancy that did.
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