In a periodic Assured or Assured Shorthold Tenancy the provisions of section 13 of the Housing Act 1988 are used to increase the rent. This is not a wholly satisfactory system as it is overly technical and ultimately allows appeals to the Rent Assessment Committee which can be somewhat capricious.
It has been thought that a clause in the agreement which set out a mechanism for increasing the rent, however abbreviated, would be sufficient to oust the provisions of section 13 and the clause would prevail.
In London District Properties Management Ltd v Goolamy  EWHC 1367 (Admin) this view has been overturned. The High Court ruled that the prevailing view was inaccurate. Taking a literal view of section 5(3) of the Act the Court held that in a statutory periodic tenancy the provisions of section 13 would overrule any rent increase clause.
Bizarrely, the legislation appears to draw a distinction between tenancies which are intended to be periodic from the outset and those which start out as fixed term tenancies and become periodic by operation of section 5. The former can incorporate rent increase clauses, the latter will have theirs overruled by the section 13 process once the tenancy has become periodic. While the Court does not mention this point it would seem that the way around the problem is to simply agree a tenancy for a fixed term with a contractual provision that it will then continue as a periodic tenancy. Presumably if it is pre-agreed that this will occur then the provisions of section 5 will not be required to create a periodic tenancy and thus the section 13 provisions will not be given the primacy that section 5(3) provides.
Whether this will work or not remains to be seen.