Ashley,
I think you're going down the right route. There are two rules that the building control officer is trying to impose, one of which is probably valid, the other may not be. These are, respectively, putting sprinklers in rooms with more than one bed and, secondly, extending the sprinklers to the rest of the building.
Wee Brian, above, has given the reason why sprinklers are desirable in a bedroom with more than one bed.
But then we come to the rule that says that sprinklers should be installed in the whole building. Clearly,in this case, the sprinklers are for a specific task - protecting an occupant of a bedroom when fire occurs in that room. If a fire were to occur elsewhere in the premises (an unsprinklered part) then, with appropriate fire separation, it makes no difference how many beds are in each room!
The reason that sprinklers should generally be installed in the whole building is because they are not capable of dealing with a fire over a certain size (for example, one that may develop in an unsprinklered part of the building) and they may be useless in such a case. Here, the sprinklers are required for a specific task in a specific part of the building and application to the remainder of the building is irrelevant.
Having said that, the installation of sprinklers in such premises should generally be encouraged. We are all aware of fire tragedies that have occurred in this sort of building. The BRE report on the effectiveness of residential sprinklers clearly indicated that the benefits of such installations outweigh the costs (when applied nationally).
And there could be benefits for the individual owner of a home fitted with sprinklers. Insurance premium rebates and attractiveness to prospective residents to name but two. There would definitely be benefits if he did have a fire!
There is a forum for battles such as you propose to be fought and that is at CLG by requesting a determination. I would recommend that.
Stu