First - yes, of course, try and find the root cause of the problem and solve that if possible.
Second - if the most practical solution is seen as changing the system to a two stage one with an investigation time, I would back up what val says. It's not the fire service that should approve the change, the fire risk assessment should be adjusted and this should demonstrate that the two stage system will continue to provide a safe building in the event of a fire.
The fire service will be in the loop but not in the way the alarm company see it.
1. They will be the ones to audit the fire risk assessment at some stage and so will be in a position to offer good advice at this stage.
2. They will be aware of the speed and weight of response to the building (through the IRMP) and will be able to advise on any drawbacks that would be introduced with the change to a two stage system.
So, yes, talk to the fire service as they will be interested and will have relevant advice, but the alarm company should be looking for a suitable and sufficient fire risk assessment that incorporates the two stage alarm as written confirmation that the system can be changed.
I'm assuming that the proposal is that the fire service do not get called until fire is confirmed or the pre-determined time limit is up (stage two). Interestingly, the standard used to be for the fire service to get called at stage one and not, as is now prevalent in an effort to reduce UWFSs, at stage two. Careful consideration needs to be given to the introduction of a two stage system, the risk assessment must examine all issues thoroughly.
Stu