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Our mortgage has an overpayment threshhold that triggers an auto-recalculation of the capital balance. Our previous monthly overpayments never triggered it. We had an option to ask, via an online 'Contact Us' page for a balance recalculation. We also had a line item on the annual mortgage statement 'Misc/adj'. Following a written request for explanation, this was explained as all the incremental reductions as the monthly balance reduced.
I may have answered my own question, my overpayment dates are random and have been at the start, middle and end of the month depending on when I bother to work out if I should over pay that month. I think I just need to be satisfied with the interest amount reducing therefore the relative overpayment taking larger and larger chunks of the capital.