Lasting Power of Attorney

My mother who has never previously had to fill in self assessment now received interest income. I have submitted my Lasting Power of Attorney to manage her financial affairs and iI have submitted this to HMRC - they confirm they have retained a copy of it. I have obtained a UTR for my mother. I need help now to set up an online account for her as her attorney. She doesn't have a passport of driving licence so I can't prove her identity. I am not sure how to navigate the process and need help with this.

Hi,
If you cannot set up the online Personal Tax account you may wish to speak to our Self Assessment team:
Self Assessment: general enquiries.
Thank you.

I've done that and sadly they can't find any guidance either. So they are sending me a paper return. It is odd though , this can't be an unusual situation so it would be good if HMRC could issue some clear guidance on this.