TUC: Responding to harmful work-related stress
Employers have a legal duty to remove, or control, risks as far as is reasonably practical. That includes the risk of work-related stress. Failure to do so can make an employer liable to fines and penalties, and even to criminal prosecution.
This means that employers have a duty to actively consider stress within 'risk assessments' and to take action to eliminate and manage it. More about stress can be found on the HSE webpages. The HSE has published Management Standards on how to do this, and there is joint TUC/HSE guidance on how to use the Standards in the workplace. The employer does not have to use the HSE's Management Standards, but if stress in a workplace is an issue, they must seek to eliminate it and be able to demonstrate that there are policies and practices to control, minimise and mitigate work-related stress.
As with all health and safety concerns union reps should seek to deal with work-related stress through 'normal channels', such as a health and safety committee, dialogue with the employer, negotiations, and grievance hearings. Interventions by an enforcement authority should be the next step for a union after other approaches to remedy an issue have failed.