agent received overpayment without informing me and filed self assessment without my knowledge

I have received a letter stating I need to file for self assessment but I did remember I sent a request not to do it for this year as I do not need it anymore. When I went through my page, I found out my agent who I have not been in contact with for almost 2 years filed it for me. Also worst, they a have received an overpayment last January. I called them about this, and they said they are waiting for notification from HMRC for the said transaction. I called HMRC and said they don't release this kind of notification. What they did for me instead is gave me an SA statement to show to my agent. I am aghast since they have filed for overpayment and self assessment without my knowledge. If i havent looked at my files I wouldnt have known about this. I emailed them with all my proof and now waiting for their reply. How else should I go forward with this if I do not receive any response from them?

Hi

This matter is between you and your agent as the onus is on you to notify any changes in circumstances and this includes letting us know that an agent is no longer acting for you.

Thanks