ESA notification letter of arrears - £6k

I received a letter from the ESA advising that they had under paid me by just short of £6k. The letter also advised that there may have been an over payment, which was being investigated. It stated that had I not been contacted within 5 days, I was to ring them. I did this twice and got nowhere, just obfuscation. I then received a phone call 2 days later from Sheffield service centre, telling me it was a mistake due to my pension increase in April. Has anyone had anything similar and should I take this further?

I have dramas every year with the annual pension increase. I just wait til my May payment before informing them, as the April one is always a mess.

I sometimes forget tbh but HMRC inform them anyway and they adjust accordingly.

I had an ‘overpayment’ one year but as I had informed them twice, I didn’t have to pay it back, as it was their error.