Joint tenancy

Rent Arrears

I have one student in a joint tenancy who has not paid this month's rent and is avoiding me. Do I send a letter by recorded delivery to the whole group to inform them and not name the student, or at this stage do I target the student in question? How many warnings do I have to give before I go to the small claims court? I am keen not to let this develop into such a large sum of money that the student won't ever be able to pay, and the rest of the group might think it worth their while not to pay the last month's rent.