Could you not enhance the use of technology?
That is a more advanced fire alarm system indicating the sources and nature of the alarm?
Perhaps cctv to identify or ensure that the areas are clear?
A more robust method of monitoring staff presence?
Swipe cards etc for access egress?

Just thoughts and questions as per usual from me!

Ron, I am guessing that the zone in alarm gives the signal to exit immediately, with other non-adjacent zones having some sort of intermittent tone until 6 minutes has elapsed, upon which the whole place goes into evac mode. Messy will correct me if I am wrong.
Messy in the past I have come across this issue. The one which worked best is leaving the day routine as it is and either increasing resources out of hours - cctv, security, upgrade to fire alarm etc, the obvious things - but most landlords won't get the tanners out to pay for this.
Best I have seen is when I worked in corporate security in a very large office block. The level of working out of hours was low, and there were 18 floors in this block. All occupied by one company, which made it a little easier.
Three security staff out of hours. One to stay in ground level control room which was adjacent to main reception to work the cctv, panels and electronic doors to let Herts Fire Service in, two to search the building. Each floor took a minute so there was no way unless you trebled security that this could be carried out in six minutes.
So overtime working was simply not allowed unless it was either a) organised in advance and security were informed accordingly or b) unplanned overtime was taking place in which case those staying behind HAD to inform who, where and for how long. Those who did not ring to tell security they were staying were found during routine inspections and were instructed to leave immediately. The process had been agreed by the client MD who agreed that this process was the only way forward and that if people were found without informing security, they would face disciplinary action.
Tried a dummy run three times and all staff staying behind were where they said they were. Went back to Herts F&R who agreed that with the procecedure/systems we had, no search would be necessary if fire alarm sounded - with certain very strict caveats and as long as we had dry runs quarterly. Lucky also in that the card entry system for all doors could show at a press of a button how many were left in the building and even which door they had last accessed. Just had to compare that with the list of those who had organised staying behind. If the lists were equal, all good, and in an out of hours evac, as the people left they still were told to swipe their cards even though the locks were released, which showed that they were out of the building.
Thinking of doing this in a building in central London which I now have taken on some fire and H&S responsibility for, but multi client, some clients more open to FM management instruction than others. I have a meeting next week to propose, will see how it goes.

Hi Messyshaw,

I can see your concerns about having only one person looking for a fire but is that based on the individual's safety or how long it would take them?

In my experience, even where you have more people they would usually split up to search for the fire and it would tend to be only one pserson that ever discovered a source. subsequently the risk to one individual would not be drastically effected by the number looking for it.

How well trained are the guards in looking for fires?

Messy, I have experience of a similar situation to yours and the answer is really a combination of technology and training:
1. Ensure that there are adequate communications between searcher and control room at all times.
2. Ensure sufficient CCTVs that automatically 'pan' onto the area involved on activation of detector head therefore providing an early visual to control room and continuous monitoring thereafter.
3. Ensure enhanced training to security staff to limit intervention in a fire situation to that which is manageable by single extinguisher with the caveat that if in doubt (e.g. noise, smell, etc, from behind a door) do not intervene - ensure doors are closed to contain the fire then confirm fire to FRS and await their arrival.
The latter point is extremely important as a suitable and sufficient FRA will suggest that one person should not tackle a developing fire (only small and contained) and that any confirmed instances of fire should always be relayed immediately to the FRS due to inconsistencies in local Unwanted Calls policies to then ensure the correct response is mobilised.

I will just add a slight aside to this. Specifically regarding radio reliability/range;

All PMR's are not created equal! You get what you pay for!