Sarguna Pandian Gunasekaran

HMRC has sent me a letter to return the overpayment made by HMRC for the tax year 2017/2018. This is the year I came into UK and filled my Tax return and I do not have my P60 or the Bank account I had then, I have closed the Bank account few years back and changed two employers from 2018. Also I was an expat under deputation at that point of time. I am not sure whether received the payments that HMRC claims or I am not able to valdiate the assessment HMRC has carried out. I am asked to payback £2000 . Any pointer on how to handle this one.