Backdating employment start date

2018-10-19 05:02:38

Is it possible to backdate employment start date and tell HMRC. I have been working part time for my current employer since april 2018 and just now I found out from employer that he hasn't told HMRC about me (when I asked him for payslips). Question is can we tell HMRC now that I have been working from April 2018? I am getting paid cash every month and would also like my payslips for every month. I and my employer is from UK.