Nigel Ash thank you for your supplementary question
You are correct that the 50:50 rules does not apply in your circumstances.
I have provided a link to the guidance at PIM1030 here PIM1030 - Introduction: jointly owned property & partnerships - HMRC internal manual
This explains that where there is no partnership, the share of any profit or loss arising from jointly owned property will normally be the same as the share owned in the property being let. Joint owners can agree a different division of profits and losses and so occasionally the share of the profits or losses will be different from the share in the property.
The share for tax purposes must be the same as the share actually agreed.
I would suggest that you get something drawn up in writing to show the beneficial interest in the income has been changed. You don’t need to provide this to us but if an enquiry was opened you would need to be able to prove the agreement.
This doesn’t mean legal ownership will change, you will still legally own the property in the proportion you do now but beneficial entitlement to the income can be changed.
You may want to get independent advice.
Thank you