Hazard wrote: Any thoughts on this issue would be much appreciated!
I can’t say I have experienced any disallowance by HMRC of interest-free balloon mortgage loans in cases similar to yours even though written documentation of the agreement has been prepared subsequent to the transaction and did not fully address all the aspects usually covered by a mortgage loan agreement.
But Maths’ lengthy analysis in his last post deserves respect and he cautions over the risk of “past consideration”.
Your specific circumstances will, I suppose, affect your decision about what to do -- e.g. how long ago was the property purchased and are there any independent witnesses (for example, a solicitor, chartered accountant, clergyman) to the verbal agreement between you and your mother who would be willing to prepare a witness statement and confirm that the terms of the written agreement are those of the verbal agreement.
But there seems no reason for you not to acquire an interest in the property, as mentioned in my penultimate post, to extinguish the liability.