using my registered lasting power of attorney

Hi all, I need to access my mother's tax details, I have an LPOA but I don't know how to proceed. I wrote to Pay As You Go, HM Revenue and Customs. BX9 1AS with a certified copy of the Lating Power of Attorney to inform them of her change of address just over a year ago. HMRC are sending her post to the new address. I am assuming HMRC have registered my LPOA, perhaps that is a foolish assumption. I don't know how to log in. I have tried to use the Goverment Gateway using my mother's details but I can't log on without various forms of ID which my mother, who is now living in a care, home no longer has. What should I do. any suggestions please. Thank you, 

Name removed .

Hi

In order to access the Personal Tax Account you will need to pass the additional verification. If you can't verify with the options provided you will not be able to access an online account for her.

Thank you