Increasing Rent

Tenancy Types and Management

I have several properties, all with great tennants.
Several years ago I signed 1 year ASTs with them. I believe these are now a statutory periodic tenancy.
As interest rates were static and the rent you could expect rose by very little in our area, I never looked for an increase in rent.
I came on to the RLA site to down load a new AST form with a view to getting them to sign a new contract at an increased rate.
However, I also found a 'section 23 rent increase' form.
What would be the most appropriate way to increase the rent?
Thank you