Dont follow that at all Inspectionofficer. How is the contractor any more at risk on the third floor landing than the other relevant persons such as the occupants of flats on the third floor?
I dont think we have directly answered the original question though. If the building is not an HMO, if it is a block of self contained rented flats what is the position?
If the building was converted to flats after 1991 then the Building Regs in England would have ensured that each flat has a mains powered smoke detector . Pre 1991 no provision was made. I suggest if the conversion was pre 1991 and the flats are let unfurnished then there is nothing more than a moral duty on the landlord to provide smoke alarms in the tenants flats, and now under the RRO, through the risk assessment, to consider an alarm and detection system covering the common areas.
I also think that if the flats are rented furnished they will be more likely to be defined as an HMO. The risk will be higher as the tenants are more likely to be short term at risk groups so any moral duty on a landlord is more compelling.