How long must we wait for an employee to participate in an Investigation where the he is providing medical certificates confirming too stressed to participate?

Unfortunately, there is no rule of thumb on how long you must wait. This will depend on the facts of each individual case and there is very little that you can do to speed the process along.

Given that medical advice agrees that he is too stressed to participate in the investigation, you may struggle to justify proceeding in spite of this advice. That said, additional stress is not necessarily a reason to postpone the proceedings (other than perhaps for a short time).

As the DWP’s Health and Work Handbook notes, where the employee is suffering from stress-related or depressive symptoms: “… it is likely that the effects of an unresolved dispute on the employee’s mental health may be greater