This is a reply from Peter Kelly OH Psychologist at HSE regarding this issue:

Jane

I hope the following clears up some of the issues surrounding stress and
     RIDDOR.

Occupational stress (work-related stress) or stress-related illnesses are
     not reportable under the Reporting of Injuries, Diseases and
Dangerous Occurrences Regulations 1995 (RIDDOR).

The question of whether to include occupational stress as a RIDDOR
     reportable disease was considered during the development of the
current Regulations. On the advice of HSC's Occupational Health Advisory
     Committee, it was not included.  This is because the causes of
stress-related ill health are usually extremely complex and linking
     conditions to specific types of work activity (as is done for other
diseases and conditions in RIDDOR) would be very difficult.

This does not mean that issues around stress-related illnesses c! annot be
raised with the enforcing authorities. Nor does it mean that a complaint
cannot be made which could be investigated. While work-related stress is
not reportable, employers have duties under the Management of Health and
Safety at Work Regulations 1999 to assess and manage the risk of
stress-related ill health arising from work activities, and under the
Health and Safety at Work etc. Act 1974, to take measures to control that
risk. This would include recording and analysing the results of any
monitoring activity.

Hope this helps