maths wrote: In the present case this does not appear to have been done and it would be difficult (if not impossible) to argue successfully that the "debt" is deductible on death.
If the transfer was not a loan then it must have been a gift -- prima facie this would make no sense in the circumstances here and so the later written agreement would serve only to confirm the prior verbal agreement at time of loan.
maths wrote: Prima facie the loan was interest free; any borrower would not now agree to an interest charge (ie a change in the terms of the loan) unless there was some quid pro quo.
As I mentioned in a previous post I’d question whether starting to charge interest is appropriate in the circumstances of OP and his mother; but in general the quid pro quo for agreeing to start paying interest is that there will be no demand for repayment of a loan whose term so far has been unspecified and whose repayment is on demand.
But without knowing the full circumstances of OP and his mother, the appropriateness to their situation of any of the ideas mentioned here is unknown – it may for example just be best for OP to acquire a part interest in the house in exchange for the loan but agree that all the net rental profits go to the mother -- see PIM 1030 http://www.hmrc.gov.uk/manuals/pimmanual/PIM1030.htm