It is a widely held belief that there is such a thing as a common law wife or husband. In fact, since the Marriage Act of 1763, there has been no such thing in England and Wales. Whether you live with your "partner", "cohabitee", "live-in-lover", "domestic associate" or "current companion" in the eyes of the law there is no special relationship and you are not equivalent to a husband or wife.
Even if a couple having been living together for several years and have children they are not regarded in law as common law husband and wife and if things do not work out a cohabitee will find that they do not have the same legal rights or safeguards as a husband or wife. However, the law is different north of the border. You may find yourself married under Scot's law even if you have not gone through a formal ceremony.
The law advises that if purchasing a property have it in joint names. But of course this is not always possible. If the injured party can prove that they contributed to the value of the property ...e.g paid for a extension they may be able to obtain that percentage of the value of the house but this would all need to be proved through the courts. However, if there is a genuine claim it would be prudent for both couples to agree with say the aid of a solicitor because the courts are the winners in the end with the costs that may have to be incurred to the battling couple!!