English law grants very few rights to people in a common law marriage. It's really important to correct the myth that common law marriage means anything because it really doesn't in England.
Intent and agreement is a big one, not length of cohabitation. The two both must consent and intend on being married and basically tell everyone they are married. Just cohabitation for a long time isn't enough. Nobody can suddenly "find" themselves in a common law marriage they didn't intend or want to enter into, they both must intentionally agree to enter into one.
http://en.wikipedia.org/wiki/Common-law_marriage_in_the_Unit...