Thanks for your reply.
As regards ***** ***** going on the Deeds, this is not essential to enable your interest in the property to be protected. If you are happy for your Partner to buy the property in his sole name, then what needs to be done is as follows;-
1. a Declaration of Trust needs to be drawn up and signed by you both.
2. a Land Registry Restriction needs to be entered by your Partner's Solicitor at the time he registers him as the new owner of the property.
The Restrction will say that no sale of the property can proceed unless a written certificate is signed by you. This Restriction therefore protects you and prevents your Partner from selling the property without your consent (which I am sure would never happen, but is always best to have this in place.
Turning to the Declaration of Trust, this will confirm that although your Partner is the sole legal owner of the Property, that he holds a share of the property on trust for.
It will also detail exactly what your financial interest in the property equates to.
You are correct that your interest should equate to 21.97% of the profit less any monies spent by your Partner.
The only tricky point is should you also be entitled to any further monies bearing in mind your mortggae payments.
Your Partner could "argue" that you are investing your money in return for a profit, and any interest payments you are having to make to enable you to put the £10,000 into the house should not also be included. This is a fair point in that he could be putting his savings into an investment elsewhere and earning interest on it from day one, but he has decided to put his money into the house, in the hope that this bears a profit.
Accordingly, normally, you wouldn't be entitled to re-claim the interest you are having to pay on the additional Mortgage payments.
You can of course discuss this with him and see if any allowance could be made in your favour, but from a neutral point of view, it would be fair for you to incur the mortgage payments yourself.
I hope this assists and answers your question.