Advice about Student Finance England

I’ve found myself in an very uncomfortable situation with Student Loans Company / Student Finance England and I would appreciate your advice as I don’t know how to proceed.

I enrolled in a part time MSc degree in October 2019.
I must have applied for a PG student loan online during my university registration, but I changed my mind during the first term and told the university I would be self funding.

I got on the phone to the SLC after receiving my first loan payment, saying that I’d like to cancel the PG loan and will be self funding.
I promptly refunded them the first payment the day I received it. In my mind, the entire case was now closed.

Now, fast forward to January 2021. I discover that I am still receiving payments; to be specific, I received a further 2 payments in April 2020 and October 2020!
And I have been accruing interest on these payments too. I was shocked. I tried telling them on the phone that this shouldn’t have happened because I was told my loan was cancelled. Now, I didn’t receive any assistance from their administrators. They plainly told me that this is my balance which is due.

I was eager to get this saga finished, and so I paid them on the 7 January 2021, the amount due for April 2020 + October 2020 - but this is where I think I’ve made my biggest mistake - I also paid a surplus amount (i.e. interest). Does this make me complicit in agreeing to their loan?

The last payment I received from SLC was on the 11 January 2021, which I promptly refunded to them. I thought that I was free from their tyranny.

Which brings us to today. I log into my online account with SLC recently, to find out I have a balance due of over £9500… this is beyond incompetence now and is actually serious.
I get on the phone to the administrators at SLC/SFE to find out what’s happened. I finally get through to their billings department. The man tells me he has found my repayments - but my repayments have gone to repaying my Undergraduate balance!

He said it will take a week for the balance to shift from the UG to PG loan. I tried to convey my shock at the situation, but obviously like the good administrator he is, he doesn’t give a sh**.

So can someone advise me what can I do now?
I am 99.9% positive that interest will have been accruing on the loan. Should I give in to injustice and repay the interest? Or is there some justice left in the world and that I have some rights to take them to court?

All I wanted was for them to stop the loan, but I feel like I am due compensation now for the way they’ve handled this entire farce of a situation.

I have clear evidence on my bank statements of A) me refunding them and B) me self funding my PG degree.