[Solved] Forced to pay Arreas they admit are incorrect

Evening all

I have sort out and resorted to this forum to seek advice. I have been paying direct to my ex partner for 6 years which is dictated anually by the CSA. In June 2018 I was made redundant. I informed them and it was all good. Payments were reduced accordingly. January 2019 mycontributon based JSA came to an end and I found work 3 months later in March 2019. I informed the CSA and they altered payments accordingly to reflect £25k/PA but only 4 months later in July, however, they informed me that there was £2100 of arreas to pay.
After contesting this and asking WHY with no answers other than I owe it following there assessments... I had to request a manditory reconsideration which came back as the arreas were infact accurate and I needed to pay.

I called up again the same day that I recieved that letter and a female said she would take ownership of this and sort it out. She came back to me 5 weeks later and said it had been calculated incorrectly as their systems had recieved a change in January from HMRC regarding JSA finishing and I had been defaulted back on to £53k/PA income for 3 months (my previous job). This person said she would correct it, apologied and I had a new letter with a new monthly payment and the arreas were removed!!

In May this year 9 months later I had my annual review and payments were slightly decreased but with NO arreas. Then two weeks later I have another letter stating Higher payments with the arreas back on. I have spent 6 weeks again ringing and ringing and contesting and waiting for call backs and letters. Last week I spoke with a chap who told me the following......
In July 2019 they had incorrect data when they sent me the arreas, now they can see at the time that I shouldnt have owed this money, however, because I didn't appeal within 30 days after the manditory reconsideration via the correct process and instead called back on the phone and allowed another colleague to resove the issue for me, I still must pay the arreas in full. I asked him straight.... you admit your data at the time was incorrect which put the arreas on the account, now 11 months later you can see that but you still want me to pay £2100 of arreas because i didnt appeal via the correct process.. and he said yes....there is nothing I can do and no-one else I can speak to about it either.

I have requested a copy of the phone call because it truely is an interesting 30 mins of hearing someone say I didnt owe it because they were wrong, but now i do owe it because I didnt follow processes. I asked him if he felt awkward or embarrassed about what he was saying... he had no empathy.

Is this for real? £2100 to be paid to my Ex who isnt even entitled to it with data evidence just because I didnt appeal when at the time the reconsideration was done they still had the wrong data? Thanks in advance.