Disputing Overpayment after termination

Short story, my former line manager sent my termination form to the wrong inbox, and even after I highlighted I was still getting paid nothing was done, this was after the pay centre said they had no record of my termination (wonder if I would have got paid to the end of my term if I had kept quiet).

How can I dispute this and put the blame on his end. Termination?overpayment letter had no information on this process.

Edit: I have contacted my MP several times over the past year about these issues and heard nothing back (Scott Reid, CON)