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Ideally, you need your fiancee to sign a Second Charge in your favour, setting out the full amount of your financial interest in the property. There would then be no argument at a later date as to your interest. However, to enter into a second Charge and to register a second charge at the Land Registry, you will need to obtain the consent of the first lender before this is done. Your fiancee's Mortgage Company may not agree to this if there is little equity.
If a second Charge is not possible, you should instruct a Solicitor to prepare a Deed of Trust for you, which will set out your financial interest in the property and which does not need to be registeered at the Land Registry, an hence your fiancee's Mortgage Company does not need to consent to any such Deed. Coupled with the signing of the Deed, you can then register a Restriction at the Land Registry which means that your fiancee won't be able to sell the property without your consent. This therefore protects your financial interest.
On a side issue, the problem you have with protecting your financial investment, is that even if you register a second Charge or a Restriction, your interst ranks below the interest of your fiancee's Mortgage Company. Therefore, there is no guarantee you will get your initial £118,000 back in the future, if there isn't this amount of equity in the property.
I hope this helps and sets out the legal position.