Have recently received an FRA which has an action relating to a cupboard containing a boiler at the head of the stairs within a rented maisonette. there are several vents within the compartment and the door is not FD30. does this need to be taken into consideration, or is it classed as a dwelling therefore the front entrance door to the common area and surrounding walls facing it should only be considered?

Depends on the scope of the FRA. If strictly for Fire Safety Order compliance then it's outside the scope of that legislation. If it was also for the purposes of the Housing Act then it may well be a necessary remedial action, but for the landlord, not the tenant.