You can't cover everything in detail in written contracts. We strongly recommend that all employers have a minimum set of HR policies and procedures and make sure that all employees know and understand them:
Performance Management policy & process
Disciplinary policy & procedure
Capability Procedure
Grievance Procedure
Time-off and absence management policy
Parental and family rights policy
Maternity, paternity & adoption policy
Retirement policy
Equality policy
Of course, there are more you could have, but this basic set will ensure that everyone knows where they stand.