Thankyou for your response.
HMRC are insisting that the repayments are rental income and therefore taxable despite explaining the situation in the same words as I have done to you.
Are you able to provide a statement clearly stating the position.
HMRC are assuming (quite understandably) that the payments are rental income because, in the absence of a loan agreement, your name being on the title deeds and your son's name not being on the title deeds, the property is yours to all intents and purposes.If you are happy to disclose the payments as rental income, deduct any allowable expenses and pay income tax on the "profit", then that's fine but it is an inefficient way of doing things. It would have been better to lend your son the cash to buy the house and to have a loan agreement drawn up with that loan being secured on the property. As it is, you have dug yourself into a hole. Had you told me initially that HMRC were on to you I would have told you what I have told you in this post as well as what I did in my initial answer.
You now have to convince HMRC that the payments are repayment of a loan.