If a block of flats has a secondary means of escape - all flats have a rear exit into the garden (and an exit to the street from there)- do the flat front doors need to be fire doors? Nobody will need to pass them, if they use the rear exits.

The block in question has an evacuation procedure, with a Grade A fire alarm system.

Thanks

In my opinion yes, all means of escape need to be separated from each other by at least 30mins fire resistance. I would not rely on a fire alarm system to guarantee a safe MoE.  

The fire rated front door needs to protect the internal MofE from a fire within the flat. 

And not all persons visiting the block have access to the rear exits.