What proportion of your personal statement should be based on extra-curricular activities?

This really depends on the University you're applying to. For Oxbridge applications, extra-curricular information should take up as little as 20% of your character count. Some Universities, for example Durham, allow you to submit an alternative personal statement, as they are more interested in extra-curricular activities. In general, the non-academic section of your personal statement should be between 1/5 and 1/3 of your total character count, depending on where you're applying.