Online self assessment with power of attorney

I hold power of attorney for my mother. This is registered with HMRC, and I have the identity documents required to register for online submission of her tax return. I have done that, and all good so far. However I have run into an issue immediately with the 'Tell us about you' page, because I cannot work out where to use my own details and where to use hers. The pre-populated information was definitely wrong in places, so I do need to make some changes. What I have so far is: Name: this was pre-populated with my name, but I have swapped it to hers. Was this the right thing to do? UTR: her UTR Address: this was pre-populated with a mashup of my street address and her care home's postcode so I need to fix it. Do I use her care home address (HMRC want to know where she lives), or mine (HMRC want to know where to send letters)? The care home are brilliant but it still isn't a secure destination for any tax-related post. Any letters need to come to my address. Date of birth: her DoB The other sections are straightforward. It is really the name and address that I am struggling with, because getting them wrong could cause follow-up problems! The chatbot can't deal with anything this complicated, the PoA section of the phoneline can't help because the problem is about filling in the return, and the self-assessment phoneline is not taking calls. Any help from a real human would be very gratefully received!

Hi Erica Bithell,
We are sorry for the issues you've experienced in completing your mother's return.
In this case, please enter your mother's name, and your mother's addres.
Any letters will still be sent to your address as you are nominated on her record to receive them.
Thank you. 

Hello The address set up for me as attorney has the same problem as reported above. My address but my mother's care home post-code. Who can I write to, to correct the post code - I cannot get through by phone. Also, as my Mum has had dementia for 5 years, she no longer has the ID for me to set up an on line account. Who can help me with that? Thanks

Hi Ian,
The address to write to is:
Pay As You Earn and Self Assessment, HM Revenue and Customs, BX9 1AS.
For assistance regarding setting up an online account, you will need to contact our online helpdesk:
Technical support with HMRC online services
If you are unable to set up an online account, then once the POA details are submitted you can contact HMRC for assistance:
Income Tax: general enquiries
Thank you