Santander credit reporting is destroying my credit score and I'm not sure what to do about it
Got a student account from Santander, £1500 overdraft. Financial situation hit pretty hard at uni and ended up having to max it out - though I paid my student finance into the account every time I got it so I met the terms of the account. Upon graduating, I was £1500 into the overdraft and I used it once mistakenly for a purchase of like £9, putting me £9 into my unarranged overdraft.
The account switched to a graduate account, which doesnt require any payments into the account like the student account, however I didnt realise I was £9 into my unarranged overdraft. Santander tried contacting me, asking if I needed financial help but I just assumed this was because I was using a lot of the overdraft and not because I was in an unarranged one. Fast forward to January, where I finally open the app and see that my account has been closed. I called santander and we set up a repayment plan for the remainder of my overdraft (which I have been regularly paying on time). I looked at my credit report and saw that 5 missed repayment reports were made in February, spanning from when my account switched to a graduate account (Mid July) to february - 5 missed repayments reported total. Since making the repayment plan I have been paying on time but I have been seeing missed repayments reported every month since the plan was made. I called santander and they said that the missed payment reports would happen every month until the overdraft is paid off in full (which was agreed in the repayment plan to take around 10 months). Nobody told me I would be reported as missing payments despite being in a payment plan, and I'm looking at probably around 15 missed payment reports on my credit file for this account because I cannot afford to pay it off any sooner.
What do I do? Is this a proportionate reporting method for what's going on? I feel like because of £9 and not being contacted explicitly by santander to say my account is in an unarranged overdraft is going to fuck up my credit file for life. I want to report this to somebody to try and sort it out but I dont know who and, more importantly, i dont even know if I have a leg to stand on given all of the technicalities they were telling me about on the phone.