A landlord of a block of flats is required to make and to keep updated a fire risk assessment in respect of communal stair cases, the only means of escape for 1st & 2nd floor residents.

If the staircase has its treads and landings covered with a synthetic material that, on heating, decomposes giving toxic fumes. Should such a latent hazard be recorded in the fire risk assessment ?

You are correct the landlord has to keep an updated fire risk assessment (FRA) in respect of the common areas and should maintain a low fire risk as possible. However the main concern is the walls and ceiling which should meet a class "O" or class 1 spread of fire, this helps to ensure the escape route maintains a low fire risk. Because this area is a low fire risk the floor covering should never be involved and a bigger danger is bad maintenance resulting in a trip hazard. Because of this it is unlikely it would be necessary to include it in the FRA unless the fire risk is increased as the result of other reasons.