Thanks for your reply.
You will need to tread carefully- the Mortgage Company will insist on you signing a letter to confirm the monies you are providing is a gift. This is because the Mortgage Company will not grant a Mortgage if a third party has a financial interest in the property. Hence, in the unfortunate event of the property ever being repossessed, the Mortgage Company would take what is owing to them and they would NOT
pay to you the sum you had put in.
So, from a Mortgage point of view, you will have no safeguard in place to protect your investment.
Likewise, and for the reason stated above, it won't be possible for you to have any Charge registered against the property to protect your investment, as the Mortgage Company will not allow this.
The only thing you can therefore do to protect your interest is if your son in law agrees to allow you to register a "Restriction" on the Deeds, which basically means that as and when the property is sold, you are notified and a separate document could be drawn up between you and your son in law confirming that you are entitled to £x upon any sale. This would at least give you some protection, but it is not cast iron, in that future events outside your son in law's control may mean that the property is sold without him having control of the proceeds- namely, if the property were repossessed or he were made bankrupt.
I hope this assists and sets out the legal position.
You would be wise to speak to a local Conveyancing Solicitor to discuss matters further.