Davo
I always recommend giving all staff basic awareness. Extinguishers are supplied for escape purposes also, and as such someone may have to use them, so should have an idea of how they work and their suitablilty for different fires. Another area I tell people to consider is; if some untrained person hurt themselves by using an extinguisher incorrectly, what would happen if they decided to make a claim?
It is suggested in the new guides also that basic familiarisation takes place for all staff and marshalls etc get more detailed training.
I would also say that the "duty to mitigate" does not just involve ringing the FRS. Apparently about 80% of fires in commercial buildings are dealt with by employees and unreported. If all those people were instructed to simply walk away from any fire then the FRS would have a whole new workload to contend with and I am sure insurers would not be happy.
The easy way to do it is to nominate marshalls, train them properly, and give the marshalls responsiblity for passing on basic use to other staff. Oh, and keep records to prove it all of course.
Now, purely on the legislative side of it. If you had a sufficient number of marshalls, and they were all sufficiently trained on extinguishers, I would only 'recommend' that the remaining staff are trained. Certain extenuating circumstances may warrant further staff training, but if you have good means of giving warning, good means of escape and good procedures/training etc then it could be included in your RA that these people should never be anywhere near a fire.