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 the purchasers are making unequal contributions to the purchase price, the mortgage repayments, or the cost of renovations and it is desired for these to be recognised on the eventual sale of the property, the property will need to be held as tenants in common. Each owner would then own a distinct share of the property. It would 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.
If there is any change in the respective contributions of the owners subsequently, it is important to make a new declaration of trust to vary the original one.
It is possible to enter a restriction with the Land Registry to ensure that the property can only be sold on the terms of the declaration of trust.