Was hoping a solicitor would give you a reply.
Not a solicitor, but did a law degree. This is a classic case, one might study as a law student.
The mortgage company determined your, ex maximum borrowing capacity.
That is why a gift of monies to cover the deposit was required.
You tried to get around this by a loan, pretending it was a gift, and what’s more put it in writing.
That was not acceptable to the mortgage company as you were a partner intending to share the benefit of the home purchase.
So a third party an Aunt was persuaded to also lie about a gift, and put it in writing in order to secure a mortgage for your partner.
And you then paid her back, so it was never a gift, it was a loan.
So your contract was with your partners Aunt which was completed on both sides.
You had no legal contract with your ex because no monies were exchanged between the two of you directly.
Setting that aside, the three of you engaged in a fraud to dupe the mortgage company. (Lots of people do this so no moral judgement from me)
I am not an expert on money laundering but it seems to me that this activity you engaged in could also be considered as such.
The effect of all of this is that you could not succeed in a claim against your ex partner in Court.
This is because of what is called the “clean hands doctrine”
You all broke the law so it is highly likely that the Court would not consider your claim on that basis alone.
The letter he has is irrelevant, to all of this save to prove dishonesty.
I feel very sorry for you.
Lesson is when it comes to money keep it above board.