Firstly, thanks for the prompt response - much faster than I expected!
I'm confused that you say "it wont invalidate the mortgage if you could reclassify it as a loan". The lender very explicitly stated that the deposit money could not be a loan and had already refused an earlier application where the deposit was borrowed.
Subsequent to that, and realising that our house purchase would otherwise fall through at the last hurdle, the parents agreed to make the money a gift (cancelling the prior agreement to borrow the money from them), and signed a declaration to that effect (the exact words were supplied by our conveyancer) -
And I [mother]
confirm as follows:-
1. The sum of £100,000 is being paid to [I and my fiancée] by way of gift and is not repayable.
2. I will not be seeking to hold a second charge or restriction against the property.
3. I will not be living at the property nor hold any beneficial interest in the property."
I believe that pretty much rules out claiming it was conditional but would love to be proved wrong...
Surely, if we were now to claim the funds were intended as a loan all along, we would be implicitly stating we had been dishonest to the lender at the time the above was signed?
I don't know if it's relevant, but wanted to also note that the gift was the whole of our deposit - we did not ourselves pay in to the purchase price (our own funds are being used for renovation).
I would also consider this question answered if you can provide a reference that my stepfather will accept, stating that a peppercorn consideration on their behalf would be sufficient to secure a contract creating (effectively) a new debt - I'll be seeing him this evening and will be trying to sway him on this. I'm not optimistic, but a solid and up-to-date reference would definitely help.