It's awhile since I re-visited my original post, so I offer a belated thanks to those who replied.
Some of you may be interested in an update.
The outcome is that the sprinkler system is now installed throughout the area to be protected and a substantial area below and around the protected area. (The protected area is a paediatric ICU and adjacent HDU ward on the third floor of existing buildings.)
It took some 15 months to convince the design team of the efficacy of the proposal, and the project was already well advanced before installation was undertaken. The client (my employer) took my proposal to an external arbitration group specially set up to consider the proposals and this group found in favour of fire suppression (in a very long report) i.e that a 'protect in place' fire strategy was justified and viable. No equivalency was proposed by the design team who felt that the fire strategy originally proposed was adequate. (SHTM 81 in Scotland makes provision for external verification and the system was used in this case, very successfully for the first time).
The system is a hybrid. It is a double knock dry system. This would not generally be considered as a life safety system, however it was designed after wide consultation to provide the maximum possible protection against false activation, and the probability of slightly delayed activation was felt to be justified on the basis that the area has a very high staff occupancy (almost 1 to 1 patients to staff in ITU and 1 to 4 in HDU) and consequently observation. The potential to deal with small incidents quickly was recognised. The sprinkler is intended to deal with significant incidents outwith the ability of staff to control it (oxygen enhanced fires, fires in enclosed rooms or spaces etc). It is the ultimate safety net for the purchase of time to conduct evacuation in situations where other methods of control are not immediately viable.
(Part of the case was that whilst fires in ITU are rare, the potential for oxygen enhancement is high and the probability of significant harm to patients and staff is also high due to significantly delayed evacuation. The potential severity and harm being the deciding issue in this case - not the frequency of event)
Interestingly, in the age of 'fire engineering', the concept of engineering was not mentioned once in all the discussions and meetings. The decision was based on a thorough risk analysis examining all the issues, including human behavioural issues related to staff capability etc.
I would like to think that this has added weight to the case for sprinklers. Of course we know they're not a panacea, but there is a place for them and we must use them where they are justified. Cost arguements are now a thing of the past. Risk arguements can no longer be defeated by accountants and accountability makes specifiers and clients nervous, that is their achilles heel. If you feel sprinklers have a place in a design proposal, make a good solid risk case; do your research, use case history and previous events, make your report thorough and don't spare the words supporting your evidence in the interest of brevity. Get it all down on paper and see your case through. It ain't easy, but it sure feels good when you are vindicated.