Sirs
I own a property that is rented and has front and rear doors with access to street and garden respectively. I have received a letter from the Managing Agency indicating i must replace external doors with fire rated ones. Is this correct interpretation of fire safety requirements?
Marcus

Final exit doors. in most cases, do not need to be fire resisting only if an external means of escape passes the door and you could not move away from it, if fire was emerging,  for example external doors opening onto a external fire staircase. You do not say what your property is, if it is flats, then the final exit doors from any flats into the common areas do need to be FD30s doors, but what you have said it most probably would not be necessary.

Following on from the question about front doors opening onto a common area that serves as a means of escape, I understand that the door must be FD30 but there is a sidelight  that I assume is part ply (lower part) and part glazed that forms part of the door frame - what should we do about this?  If I change the glass for fire rated frosted glass in the sidelight can I leave the existing door that is not a fire door? TIA for any help.

No, if the door, wall and associated glazing and partitions are required to be fire rated then all should be, not just some.