Thanks for the response.
Reading through the document, am I right in saying that although the building has been split into three separate residentuial units, as each unit has its own independent external entrance (onto the street), there are no communal areas and the first floor flat is less than 4.5m from the ground, they would each be classed as a dwellinghouse rather than a flat with regards Section 2 means of escape regulations?
If classed as dwellinghouses, I can't see anything definitive saying that all doors in the upstairs flat need to be fire doors unless I'm missing something?
All habitable rooms have escape windows and as said above, all units have indepenedent external entrances and below 4.5m.
You say all rooms in the upstairs flat now need to be fire doors, is that because to get access to the external entrance you have to go through the lounge?