Most landlords issue an AST for a fixed term - normally 6 months. At the end of the term the AST remains in place, but becomes a Statutory Periodic - unless the AST is renewed.
This has never been an issue, but as aresult of two separate court cases last year, this now means:-
S.21(2) of the 1988 Act provides that a notice under s.21(1)(b) "may be given before or on the day on which the tenancy comes to an end"
i.e. a S21 (1)(b) applies at any time in the life of a Fixed term AST, even if it has become Statutory Periodic. (Spencer v Taylor)
It is worth noting that you do not need to issue a Fixed Term agreement, it is possible to issue an AST without a term stated.
This would then be a Contractual Periodic AST, and a Section 21 (4)(a) would then have to be used.
Contractual Periodic makes the deposit protection slightly easier, but the S21 eviction notice far more complicated - swings and roundabouts - your choice.