Tenant objects to essential maintenance being carried out

Maintenance

I recently asked for a fire safety assessment and the resulting recommendation was for all bedrooms in a property to have fire alarms fitted. I have arranged for this to be completed and on informing the tenants of the requirements for this work to be completed and having given notice that access to rooms would be required, one tenant has objected, refusing admission to his room between 8 am and 5PM as he works nights. The contractors are only prepared to work between 9am and 5pm although they have agreed to leave work in and above his room until after 3pm. As this is a safety item and essential maintenance,what is my position? I believe that it is an offence to interfere with a Landlord in performance of his legal duties, to whom should I complain if he will not permit the work to go ahead?