Land Registry Restriction - advice on type of standard form and wording of form

Advice required on what type of Standard Form Restriction needs to be registered with HMLR to protect an unmarried partner's beneficial interest in a property. Property is at risk of repossession by mortgagee and partner has a minimum 27% interest by virtue of mortgage payments and building works. Property is owned by the other partner in the relationship and the mortgage is in their name only.

Responses from people with property law experience in England and Wales only please.