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Not sure if you are going to like my answer, but here goes-
Your son's Mortgage Lender will require confirmation that no third party has any financial interest in the property, or they are unlikely to offer a Mortgage to your son. Hence why they require you to sign a form/simple letter confirming that the money you are giving your son is a gift.
Likewise, your son's Solicitor, will be under a duty to notify the Mortgage Lender if
he finds out that the money you are giving is some sort of loan/or is potentially repayable.
Therefore, you are somewhat stuck to protect your financial interest, as it will jeopardise your son's Mortgage.
You will still be able to protect your interest, however, provided you are happy to do this shortly AFTER your son has completed his purchase.
A Declaration of Trust can be prepared, setting out the fact that the £250,000 has been provided by you and that upon any sale, this sum is repayable.This document will be signed by you and your sonand will be evidence of your financial interest. To protect you further a Restriction can be registered at the Land Registry, meaning that your consent will be required, before any future sale of the property by your son can be completed.
Please note that you will need to instruct a different Solicitor to the one acting for your son.
Please also note that even once the Declaration and Restriction have been completed, your financial interest ranks behind the Mortgage Lender's- eg if the worst came to the worst and the house was repossessed, the Lender would be entitled to their money back first, to include any further mortgage advance which your son may obtain at a later date from the Bank and you would only be entitled to "claim" your £250,000 from the remaining equity.
I hope this clarifies the situation and sets out the legal position.