Miladys negligence affects credit score

I have paid my Miladys account ON TIME however used my account number as a reference instead of their reference number. I've emailed the POP to Miladys on the day I made the payment and twice thereafter. Call centre agent calls me and confirms that they have received the payment and asks me to change the reference number on my banking app which I did immediately. A week later and still not resolved. This has now impacted my credit score which I have worked hard to maintain. I want my credit score corrected and this account to be sorted for goodness sake.