We live on the ground floor of a 3 storey purpose built block. All the homeowners have been contacted by the management company and told that they are legally required to replace their own front doors (which on each floor lead off a communal hallway) to a certain fire standard. The cost to be born by each owner. I recently heard that blocks under 4 storeys high are excempt. Or is this the responsibility of the management company?

The common areas of all blocks of flats in England and Wales are subject to The Regulatory Reform (Fire Safety) Order 2005 and require a fire risk assessment to be carried out. In most FRA,s the front door to each flat is required a FD30s fire door.

How it is financed is not the concern of the FSO that is up to your tenancy agreements so this is a legal matter  between the freeholder/owners/management/company and yourselves.

No exemptions on size, that's an urban myth I'm afraid.