Discuss Is the law on “stress at work” yet certain enough for employers to understand their duty to employees?

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[Revision] “… what was said in Hatton was not intended to cover all the infinitely variable facts that are likely to arise in stress at work cases.” Hartman v South Essex Mental Health and Community Care NHS Trust [2005] IRLR 293 at 299 (judgement of the court) Is the law on “stress at work” yet certain enough for employers to understand their duty to employees?