Hi
Thank you for your question.
Question 1 - You do not need to submit a form 17 if the property is not jointly owned.
You will not need to submit a copy of the deed of assignment, however you should keep one safe if it is requested by HMRC.
Question 2 - The Deed needs to clearly show the split both parties names, signed by both parties.
It should be witnessed and have an effective start from date.
Question 3- Expenses such as running costs and maintenance can be apportioned between both parties from the effective from date.
However the mortgage interest can only be claimed by yourself if the mortgage is in your name and you are the 'owner' of the loan.
You cannot apportion mortgage interest relief unless the mortgage is in joint names.
Please refer to this link in the savings and investment manual.
Relief for interest paid: general conditions: the claimant