If a property is purchased in joint names, this can either be held on a joint tenancy or on a tenancy in common. If the property is held on a joint tenancy, the owners do not have distinct shares and the property would automatically pass to the survivor.
If you are making unequal contributions to the purchase price, the mortgage repayments or work to the property and you want these to be recognised on the eventual sale or would not want your share of the property to pass to the other joint owner, you will need to hold the property as tenants in common. Each owner would then own a distinct share in the property. It would then also be advisable to prepare a declaration of trust. This document will outline your respective interests in the property and protect any initial or future contributions to the property.
The declaration will only be relied upon in the event that the owners cannot agree on their interests if the property is sold in the future and the terms can be varied if the situation changes. It is not a public document and does not need to be disclosed to the Land Registry. However, if the land is registered it is possible to enter a restriction with the Land Registry.