If you are conducting a risk assessment then you should be utilising that nice government document "fire safety an employers guide". Within that, corridors longer then 30 metres, the same figure appears in anumber of other guidance publications and in the new Guidance Documents for RR(FS), which incidentally will be out next week, have to be sub divided by fire or smoke doors. If the corridor is less than that figure or just over that figure why bother.
If the whole thing was missed at design stage then the FRS could not apply anything other than that as they would have had an opportunity at consultation stage to do something about it. The Statutory Bar will apply and the only way to do something now, if necessary is under the WFPL.