In relation to Safety Representatives appointed by Trade Unions you do have an obligation to assist them in carrying out their functions, and this does include providing them with risk assessments, allowing them time to provide you with their feedback and taking into account their views before making any changes to working practices which impact upon the health, safety and welfare of employees.
The relevant Trade Union should inform you about who the appointed Safety Representative is and which members/employees they represent. This will allow you to enter into dialogue with the Safety Representative.