HMRC insists I am dead. How do I convince it I’m not?

It allocated my NI number to a stranger who has died, and will not process my pension top-up request as a result

HM Revenue and Customs allocated my national insurance (NI) number to a stranger who has since died. It therefore now insists that I am dead and so will not process my pension top-up request.

I’ve had this number since 1991 when I moved to the UK for six years to work.