Thanks for the reply.
When you say "write to HMRC", do you mean I need to register the declaration of trust using the online service (https://www.gov.uk/guidance/register-a-trust-as-a-trustee), or do I just need to post the declaration of trust document to HMRC?
If I need to post the document, which specific department do I need to address it to?
I have a couple of joint savings accounts with my mum. These accounts are held in UK banks. All the funds in the accounts belong to my mums, and the only beneficiary of the interest generated by those accounts is my mum. I am only on the account to help her manage them.
I understand the tax owed for the interest generated will be automatically split 50/50. I want to understand how I can change that split so my mum pays all the taxes, as the funds are hers and she is the only beneficiary of the interest. Can the bank change the split or is there something I can submit to HMRC to inform about this situation? I've noticed there is a form that can be sent to change the split between spouses/partners, but not between family members.