Written statements of employment should be provided no later than the first day of employment, and ideally before this.
This document form the basis of the legal relationship between you and your employee. If you do want to change it at a later date, then you will need your employee's consent. It is therefore vital to get it right first time.
In April 2020 new legislation confirmed the basic information that should be included in this document. However you will also want to ensure that you maintain flexibility and therefore additional clauses including mobility and Payment in Lieu of notice are important.