Pensioner has no ID documents for Government Gateway Account

My mother is aged 90, who lives in a care home. I have complete POA, so I do all of her paperwork. She has several private pensions that I need to declare for tax purposes for her but cannot provide the relevant ID for HMRC Government Gateway account. what do I do?

Hi,
 
You will need to write to us with the information. See link below for guidance:

Income Tax: general enquiries

Thank you.