Question aboutn AST and possession

Tenancy Types and Management

In the AST it states that 'we may repossess the property if: you fail to pay us rent 14 days after it is due, whether you have been asked for it or not.' my landlord who has a tenant that is two weeks in arrears and seems to have abandoned the property has read this and wants to know why we can't evict him based on this statement. I have advised that a section 8 can only be served under rent arrears when the tenant is two months in arrears. However i would like to know where this 'two weeks' clause comes from and relates to please? It is causing some confusion with my landlord. Many thanks