Hi
You are classed as a Letting agent as you are acting on behalf of a non resident landlord in connection with the management of their UK rental business.
Your responsibilities are to register for the non-resident landlord scheme, provide a quarterly return and payment and if you need to account for tax, provide the non resident landlord with the corresponding certificate.
Further guidance is available at:
What the Non-resident Landlords Scheme is and
Which agents should operate the Non-resident Landlords Scheme
The onus , in the case you mention, is on the owner to distribute the income under the terms of the Declaration Of Trust
Thank you