If you are putting unequal amounts into the property the person who is paying the larger amount can protect that money by a "trust deed".
This is a legal document which sets out who has put what into the property and it outlines who will get what from the proceeds of the property in the event of separation.
For example, it can state that if the house is sold, the person who put in the deposit gets that amount first, before the remainder of the value is shared out.