To avoid any suggestion of bias or suspicion that your manager has already made his or her mind up, your grievance should be heard by a manager who has not been involved in the subject matter of the grievance. Where the grievance is against your line manager, it should ideally be heard by a manager who is both independent and more senior.
In a very small organisation, it might be difficult in practice to organise for your grievance to be heard by a manager who hasn’t been involved in its subject matter. In these circumstances, the person hearing the grievance should remind themselves of the need to remain impartial and to listen fairly to both sides.
Sometimes an external organisation, such as Acas, can become involved. Acas offers a mediation service for in-work disputes. Acas and the TUC have published joint guidance for reps on workplace mediation (PDF, 1MB).