If you live with a spouse or civil partner and have income from property you jointly own, by default the income will be allocated evenly between you. In contrast, if you are unmarried you are free to select the appropriate profit allocation you see fit.
However, there is Form 17 which allows married couples and civil partners to split the income according to ownership. You will need evidence to support your decision to make the share uneven though, such as a declaration or deed.
If you’d like to discuss this further, please do not hesitate to contact your local TaxAssist Accountant. They can discuss your affairs with you, complete the form on your behalf and may be able to put you in touch with a local solicitor to draw up a declaration or deed.