Jayy:

As for this being my own fault, I am well aware of that thanks.

I'm not trying to apportion blame but to understand the mechanism at work. You used the term "early" but the payment (as Huckster suspects or suggests) was either late for the August due date (probably around 28/08) or it was an early payment for September which was registered before the September statement date. Only you know the answer to that and if it was a mistake, how it came to happen. If the former why the rant about "penalised for paying too early". If the latter try your charm on the Halifax complaints procedure.

In responding to posts I always hope that others will learn something even if its not what the OP wants to hear. In this case I repeat the learning point to avoid any chance of inadvertantly not paying or paying late or paying early, make the minimum payment by direct debit. Another point to note is that the payment must have been a Faster Payment as it has been credited same day. The "allow 4 days" is to ensure that BACS payments arrive by the due date.