Renting to family members

Hi, if parents purchase a property for their children to live in via a mortgage (family occupancy agreed with lender) and the children pay the parents just enough "rent" to cover the mortgage payment, is this classed as taxable rental income on the parents if there is no formal rental agreement? If the money received is classed as taxable property income on the parents A: Can the parents claim the property allowance? B: If the parents don't want to claim property allowance can they instead claim allowable property expenses (excluding finance costs) up to the value of the rental income received? C: do the normal restrictions around tax relief for finance costs apply? Thank you

Hi,
You can see guidance here:
PIM2130 - Deductions: main types of expense: properties not let at a commercial rent
Thank you.